ClangImporter: avoid infinite recursion if we can't find a Swift declaration of a mapped type
ClangImporter: avoid infinite recursion if we can't find a Swift declaration of a mapped type

This would come up on Linux when the dispatch overlay was not built,
because importing the typedef for dispatch_block_t would try to find
perform a lookup of a declaration with the same name.

Progress on <rdar://problem/23315750>.

mightydeveloper/swift,rudkx/swift,austinzheng/swift,gmilos/swift,jopamer/swift,felix91gr/swift,tkremenek/swift,jtbandes/swift,airspeedswift/swift,jmgc/swift,LeoShimonaka/swift,lorentey/swift,dduan/swift,stephentyrone/swift,CodaFi/swift,JaSpa/swift,brentdax/swift,sschiau/swift,JGiola/swift,return/swift,kentya6/swift,gmilos/swift,parkera/swift,Ivacker/swift,gottesmm/swift,calebd/swift,parkera/swift,gottesmm/swift,nathawes/swift,amraboelela/swift,swiftix/swift,ken0nek/swift,russbishop/swift,JGiola/swift,MukeshKumarS/Swift,kperryua/swift,Jnosh/swift,emilstahl/swift,shajrawi/swift,gregomni/swift,sschiau/swift,IngmarStein/swift,manavgabhawala/swift,mightydeveloper/swift,tinysun212/swift-windows,zisko/swift,therealbnut/swift,cbrentharris/swift,swiftix/swift,cbrentharris/swift,shahmishal/swift,dduan/swift,uasys/swift,khizkhiz/swift,ken0nek/swift,shahmishal/swift,kusl/swift,modocache/swift,deyton/swift,frootloops/swift,danielmartin/swift,therealbnut/swift,manavgabhawala/swift,tkremenek/swift,benlangmuir/swift,OscarSwanros/swift,felix91gr/swift,swiftix/swift.old,jopamer/swift,deyton/swift,johnno1962d/swift,harlanhaskins/swift,alblue/swift,xwu/swift,sdulal/swift,practicalswift/swift,JaSpa/swift,djwbrown/swift,zisko/swift,calebd/swift,dduan/swift,milseman/swift,slavapestov/swift,alblue/swift,OscarSwanros/swift,codestergit/swift,kstaring/swift,hughbe/swift,sdulal/swift,gregomni/swift,cbrentharris/swift,allevato/swift,glessard/swift,lorentey/swift,austinzheng/swift,sdulal/swift,lorentey/swift,benlangmuir/swift,jckarter/swift,cbrentharris/swift,arvedviehweger/swift,rudkx/swift,atrick/swift,harlanhaskins/swift,sschiau/swift,CodaFi/swift,slavapestov/swift,bitjammer/swift,emilstahl/swift,sdulal/swift,return/swift,natecook1000/swift,huonw/swift,practicalswift/swift,practicalswift/swift,gmilos/swift,khizkhiz/swift,Jnosh/swift,tinysun212/swift-windows,ken0nek/swift,xwu/swift,Jnosh/swift,OscarSwanros/swift,tjw/swift,therealbnut/swift,shahmishal/swift,manavgabhawala/swift,tardieu/swift,apple/swift,uasys/swift,huonw/swift,hooman/swift,mightydeveloper/swift,tjw/swift,sschiau/swift,gribozavr/swift,Ivacker/swift,SwiftAndroid/swift,parkera/swift,brentdax/swift,russbishop/swift,mightydeveloper/swift,SwiftAndroid/swift,jtbandes/swift,amraboelela/swift,JaSpa/swift,devincoughlin/swift,swiftix/swift.old,arvedviehweger/swift,frootloops/swift,tkremenek/swift,OscarSwanros/swift,MukeshKumarS/Swift,bitjammer/swift,CodaFi/swift,devincoughlin/swift,deyton/swift,shajrawi/swift,felix91gr/swift,mightydeveloper/swift,parkera/swift,jckarter/swift,sdulal/swift,airspeedswift/swift,LeoShimonaka/swift,jckarter/swift,tardieu/swift,CodaFi/swift,IngmarStein/swift,aschwaighofer/swift,jmgc/swift,Ivacker/swift,arvedviehweger/swift,gribozavr/swift,natecook1000/swift,deyton/swift,OscarSwanros/swift,karwa/swift,airspeedswift/swift,codestergit/swift,apple/swift,adrfer/swift,xedin/swift,ahoppen/swift,JGiola/swift,LeoShimonaka/swift,practicalswift/swift,hughbe/swift,KrishMunot/swift,JaSpa/swift,dduan/swift,kstaring/swift,nathawes/swift,shahmishal/swift,SwiftAndroid/swift,cbrentharris/swift,hughbe/swift,jopamer/swift,lorentey/swift,gottesmm/swift,russbishop/swift,practicalswift/swift,modocache/swift,russbishop/swift,therealbnut/swift,ben-ng/swift,roambotics/swift,gregomni/swift,allevato/swift,ahoppen/swift,codestergit/swift,glessard/swift,ben-ng/swift,jtbandes/swift,calebd/swift,huonw/swift,bitjammer/swift,brentdax/swift,tjw/swift,tinysun212/swift-windows,swiftix/swift.old,karwa/swift,shajrawi/swift,natecook1000/swift,natecook1000/swift,khizkhiz/swift,kusl/swift,allevato/swift,devincoughlin/swift,amraboelela/swift,jopamer/swift,kstaring/swift,shahmishal/swift,stephentyrone/swift,tardieu/swift,swiftix/swift.old,kperryua/swift,harlanhaskins/swift,felix91gr/swift,brentdax/swift,johnno1962d/swift,slavapestov/swift,stephentyrone/swift,nathawes/swift,lorentey/swift,natecook1000/swift,xedin/swift,sschiau/swift,hooman/swift,MukeshKumarS/Swift,sschiau/swift,harlanhaskins/swift,aschwaighofer/swift,bitjammer/swift,calebd/swift,tkremenek/swift,milseman/swift,atrick/swift,gribozavr/swift,khizkhiz/swift,danielmartin/swift,karwa/swift,frootloops/swift,zisko/swift,gribozavr/swift,stephentyrone/swift,glessard/swift,kentya6/swift,practicalswift/swift,zisko/swift,sdulal/swift,arvedviehweger/swift,benlangmuir/swift,ben-ng/swift,hooman/swift,kusl/swift,kperryua/swift,IngmarStein/swift,modocache/swift,Ivacker/swift,milseman/swift,aschwaighofer/swift,johnno1962d/swift,tinysun212/swift-windows,KrishMunot/swift,khizkhiz/swift,modocache/swift,jtbandes/swift,swiftix/swift,gottesmm/swift,devincoughlin/swift,swiftix/swift.old,Ivacker/swift,OscarSwanros/swift,khizkhiz/swift,JaSpa/swift,airspeedswift/swift,xedin/swift,sschiau/swift,MukeshKumarS/Swift,danielmartin/swift,parkera/swift,tinysun212/swift-windows,atrick/swift,swiftix/swift.old,hooman/swift,jopamer/swift,dreamsxin/swift,karwa/swift,codestergit/swift,tjw/swift,hughbe/swift,tardieu/swift,hughbe/swift,hughbe/swift,adrfer/swift,apple/swift,benlangmuir/swift,ahoppen/swift,karwa/swift,frootloops/swift,modocache/swift,jmgc/swift,shajrawi/swift,JGiola/swift,cbrentharris/swift,swiftix/swift,alblue/swift,gregomni/swift,tinysun212/swift-windows,jckarter/swift,IngmarStein/swift,gmilos/swift,allevato/swift,aschwaighofer/swift,gregomni/swift,jmgc/swift,tkremenek/swift,tjw/swift,rudkx/swift,shajrawi/swift,huonw/swift,amraboelela/swift,kusl/swift,hughbe/swift,return/swift,devincoughlin/swift,airspeedswift/swift,devincoughlin/swift,KrishMunot/swift,danielmartin/swift,gottesmm/swift,zisko/swift,frootloops/swift,sdulal/swift,emilstahl/swift,alblue/swift,danielmartin/swift,aschwaighofer/swift,manavgabhawala/swift,benlangmuir/swift,kstaring/swift,devincoughlin/swift,aschwaighofer/swift,JaSpa/swift,swiftix/swift,atrick/swift,stephentyrone/swift,roambotics/swift,emilstahl/swift,ben-ng/swift,adrfer/swift,codestergit/swift,mightydeveloper/swift,shahmishal/swift,sdulal/swift,KrishMunot/swift,gottesmm/swift,swiftix/swift.old,kentya6/swift,hooman/swift,ben-ng/swift,kentya6/swift,manavgabhawala/swift,KrishMunot/swift,Ivacker/swift,jtbandes/swift,roambotics/swift,calebd/swift,IngmarStein/swift,therealbnut/swift,dduan/swift,gmilos/swift,adrfer/swift,xedin/swift,djwbrown/swift,felix91gr/swift,bitjammer/swift,arvedviehweger/swift,nathawes/swift,Jnosh/swift,shajrawi/swift,parkera/swift,jopamer/swift,kentya6/swift,atrick/swift,stephentyrone/swift,ben-ng/swift,russbishop/swift,russbishop/swift,therealbnut/swift,SwiftAndroid/swift,frootloops/swift,KrishMunot/swift,kstaring/swift,kperryua/swift,johnno1962d/swift,kentya6/swift,emilstahl/swift,LeoShimonaka/swift,cbrentharris/swift,ahoppen/swift,modocache/swift,natecook1000/swift,slavapestov/swift,SwiftAndroid/swift,emilstahl/swift,calebd/swift,mightydeveloper/swift,emilstahl/swift,apple/swift,mightydeveloper/swift,JGiola/swift,austinzheng/swift,gribozavr/swift,SwiftAndroid/swift,johnno1962d/swift,Jnosh/swift,SwiftAndroid/swift,CodaFi/swift,adrfer/swift,amraboelela/swift,austinzheng/swift,lorentey/swift,deyton/swift,danielmartin/swift,natecook1000/swift,apple/swift,tardieu/swift,deyton/swift,felix91gr/swift,bitjammer/swift,MukeshKumarS/Swift,JGiola/swift,OscarSwanros/swift,johnno1962d/swift,gregomni/swift,djwbrown/swift,swiftix/swift,harlanhaskins/swift,karwa/swift,roambotics/swift,roambotics/swift,dduan/swift,kperryua/swift,shajrawi/swift,apple/swift,tardieu/swift,gribozavr/swift,emilstahl/swift,amraboelela/swift,ben-ng/swift,kentya6/swift,MukeshKumarS/Swift,devincoughlin/swift,shahmishal/swift,jtbandes/swift,djwbrown/swift,djwbrown/swift,KrishMunot/swift,swiftix/swift,uasys/swift,stephentyrone/swift,allevato/swift,lorentey/swift,milseman/swift,nathawes/swift,tjw/swift,karwa/swift,aschwaighofer/swift,xedin/swift,brentdax/swift,jckarter/swift,gmilos/swift,brentdax/swift,glessard/swift,practicalswift/swift,modocache/swift,benlangmuir/swift,jckarter/swift,CodaFi/swift,LeoShimonaka/swift,brentdax/swift,Jnosh/swift,xwu/swift,glessard/swift,jtbandes/swift,xwu/swift,alblue/swift,IngmarStein/swift,xedin/swift,johnno1962d/swift,manavgabhawala/swift,alblue/swift,codestergit/swift,djwbrown/swift,xwu/swift,milseman/swift,rudkx/swift,adrfer/swift,lorentey/swift,danielmartin/swift,amraboelela/swift,gmilos/swift,tjw/swift,shajrawi/swift,frootloops/swift,kstaring/swift,xedin/swift,dreamsxin/swift,cbrentharris/swift,allevato/swift,arvedviehweger/swift,huonw/swift,roambotics/swift,uasys/swift,codestergit/swift,kperryua/swift,kentya6/swift,tinysun212/swift-windows,calebd/swift,slavapestov/swift,ken0nek/swift,ken0nek/swift,milseman/swift,shahmishal/swift,manavgabhawala/swift,slavapestov/swift,kstaring/swift,gottesmm/swift,huonw/swift,uasys/swift,jmgc/swift,russbishop/swift,slavapestov/swift,karwa/swift,bitjammer/swift,arvedviehweger/swift,CodaFi/swift,kusl/swift,uasys/swift,austinzheng/swift,airspeedswift/swift,ken0nek/swift,zisko/swift,IngmarStein/swift,jmgc/swift,xwu/swift,jmgc/swift,gribozavr/swift,glessard/swift,gribozavr/swift,return/swift,return/swift,huonw/swift,Ivacker/swift,return/swift,dduan/swift,austinzheng/swift,austinzheng/swift,allevato/swift,MukeshKumarS/Swift,therealbnut/swift,nathawes/swift,deyton/swift,ahoppen/swift,hooman/swift,practicalswift/swift,kusl/swift,return/swift,djwbrown/swift,jckarter/swift,kusl/swift,LeoShimonaka/swift,JaSpa/swift,tkremenek/swift,xwu/swift,tardieu/swift,kperryua/swift,LeoShimonaka/swift,nathawes/swift,swiftix/swift.old,ken0nek/swift,parkera/swift,milseman/swift,xedin/swift,rudkx/swift,airspeedswift/swift,harlanhaskins/swift,zisko/swift,kusl/swift,uasys/swift,khizkhiz/swift,felix91gr/swift,harlanhaskins/swift,tkremenek/swift,Jnosh/swift,Ivacker/swift,rudkx/swift,LeoShimonaka/swift,hooman/swift,adrfer/swift,jopamer/swift,ahoppen/swift,atrick/swift,parkera/swift,sschiau/swift,alblue/swift