[frontend] Added support in FrontendOptions for denoting the primary input.
[frontend] Added support in FrontendOptions for denoting the primary input.

When a primary input is specified, output will only be generated for that input. (If a primary input is not specified, output will be generated for the whole module.)
This commit only adds the storage for the primary input; the primary input is not yet honored by the frontend, nor is it possible to specify the primary input.

Swift SVN r12148

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