IRGen: Allow pure-ObjC refcounting class existentials
IRGen: Allow pure-ObjC refcounting class existentials

Class-constrained existentials usually have unknown
reference counting in Swift, because they can contain
either Objective-C or native Swift classes.

But if the class existential has a superclass bound
which is known to be an imported Objective-C class,
we can use Objective-C reference counting instead of
unknown reference counting.

This is tested with the next commit that adds
ClangImporter support.

frootloops/swift,karwa/swift,OscarSwanros/swift,austinzheng/swift,arvedviehweger/swift,frootloops/swift,Jnosh/swift,karwa/swift,brentdax/swift,milseman/swift,rudkx/swift,ahoppen/swift,jopamer/swift,austinzheng/swift,amraboelela/swift,stephentyrone/swift,JGiola/swift,djwbrown/swift,shahmishal/swift,harlanhaskins/swift,danielmartin/swift,aschwaighofer/swift,karwa/swift,frootloops/swift,atrick/swift,sschiau/swift,nathawes/swift,airspeedswift/swift,milseman/swift,atrick/swift,shajrawi/swift,gribozavr/swift,shahmishal/swift,milseman/swift,jopamer/swift,felix91gr/swift,harlanhaskins/swift,parkera/swift,zisko/swift,sschiau/swift,return/swift,nathawes/swift,return/swift,allevato/swift,benlangmuir/swift,zisko/swift,stephentyrone/swift,frootloops/swift,natecook1000/swift,danielmartin/swift,hooman/swift,lorentey/swift,practicalswift/swift,rudkx/swift,shahmishal/swift,rudkx/swift,aschwaighofer/swift,harlanhaskins/swift,allevato/swift,sschiau/swift,Jnosh/swift,zisko/swift,karwa/swift,roambotics/swift,practicalswift/swift,tkremenek/swift,huonw/swift,devincoughlin/swift,hooman/swift,xwu/swift,felix91gr/swift,amraboelela/swift,alblue/swift,apple/swift,huonw/swift,tjw/swift,stephentyrone/swift,brentdax/swift,gribozavr/swift,nathawes/swift,zisko/swift,parkera/swift,felix91gr/swift,calebd/swift,jckarter/swift,jckarter/swift,calebd/swift,calebd/swift,benlangmuir/swift,practicalswift/swift,devincoughlin/swift,tjw/swift,tjw/swift,shahmishal/swift,sschiau/swift,brentdax/swift,milseman/swift,jopamer/swift,OscarSwanros/swift,OscarSwanros/swift,return/swift,gregomni/swift,xedin/swift,alblue/swift,shajrawi/swift,return/swift,apple/swift,lorentey/swift,jckarter/swift,OscarSwanros/swift,devincoughlin/swift,tkremenek/swift,jtbandes/swift,aschwaighofer/swift,shajrawi/swift,uasys/swift,shahmishal/swift,JGiola/swift,gregomni/swift,gribozavr/swift,ahoppen/swift,jmgc/swift,benlangmuir/swift,danielmartin/swift,CodaFi/swift,practicalswift/swift,xedin/swift,frootloops/swift,harlanhaskins/swift,glessard/swift,hughbe/swift,hughbe/swift,gribozavr/swift,OscarSwanros/swift,lorentey/swift,benlangmuir/swift,gribozavr/swift,djwbrown/swift,tkremenek/swift,benlangmuir/swift,austinzheng/swift,amraboelela/swift,swiftix/swift,JGiola/swift,arvedviehweger/swift,alblue/swift,deyton/swift,alblue/swift,parkera/swift,return/swift,atrick/swift,sschiau/swift,swiftix/swift,allevato/swift,austinzheng/swift,nathawes/swift,tkremenek/swift,gribozavr/swift,zisko/swift,return/swift,xwu/swift,jopamer/swift,lorentey/swift,swiftix/swift,natecook1000/swift,airspeedswift/swift,karwa/swift,deyton/swift,deyton/swift,natecook1000/swift,aschwaighofer/swift,glessard/swift,frootloops/swift,apple/swift,hughbe/swift,CodaFi/swift,shajrawi/swift,ahoppen/swift,huonw/swift,shahmishal/swift,parkera/swift,djwbrown/swift,jopamer/swift,milseman/swift,tjw/swift,tjw/swift,lorentey/swift,felix91gr/swift,Jnosh/swift,gregomni/swift,hooman/swift,tjw/swift,Jnosh/swift,xedin/swift,parkera/swift,hooman/swift,amraboelela/swift,parkera/swift,karwa/swift,swiftix/swift,milseman/swift,natecook1000/swift,JGiola/swift,tjw/swift,sschiau/swift,devincoughlin/swift,jckarter/swift,tkremenek/swift,gribozavr/swift,nathawes/swift,sschiau/swift,swiftix/swift,djwbrown/swift,alblue/swift,xedin/swift,ahoppen/swift,jtbandes/swift,shahmishal/swift,djwbrown/swift,roambotics/swift,Jnosh/swift,practicalswift/swift,allevato/swift,uasys/swift,ahoppen/swift,Jnosh/swift,brentdax/swift,hughbe/swift,alblue/swift,benlangmuir/swift,shahmishal/swift,roambotics/swift,shajrawi/swift,return/swift,danielmartin/swift,uasys/swift,glessard/swift,allevato/swift,felix91gr/swift,xedin/swift,danielmartin/swift,deyton/swift,sschiau/swift,deyton/swift,alblue/swift,amraboelela/swift,hooman/swift,tkremenek/swift,devincoughlin/swift,milseman/swift,CodaFi/swift,stephentyrone/swift,airspeedswift/swift,allevato/swift,zisko/swift,jmgc/swift,arvedviehweger/swift,CodaFi/swift,swiftix/swift,aschwaighofer/swift,shajrawi/swift,atrick/swift,nathawes/swift,arvedviehweger/swift,deyton/swift,jopamer/swift,OscarSwanros/swift,atrick/swift,devincoughlin/swift,jckarter/swift,natecook1000/swift,jtbandes/swift,roambotics/swift,zisko/swift,rudkx/swift,hooman/swift,practicalswift/swift,brentdax/swift,JGiola/swift,lorentey/swift,airspeedswift/swift,arvedviehweger/swift,xwu/swift,huonw/swift,xwu/swift,amraboelela/swift,calebd/swift,jckarter/swift,calebd/swift,xwu/swift,jmgc/swift,tkremenek/swift,arvedviehweger/swift,devincoughlin/swift,karwa/swift,gregomni/swift,rudkx/swift,aschwaighofer/swift,huonw/swift,practicalswift/swift,parkera/swift,uasys/swift,CodaFi/swift,harlanhaskins/swift,stephentyrone/swift,practicalswift/swift,roambotics/swift,djwbrown/swift,airspeedswift/swift,hughbe/swift,xwu/swift,glessard/swift,uasys/swift,uasys/swift,lorentey/swift,airspeedswift/swift,apple/swift,xwu/swift,gribozavr/swift,CodaFi/swift,frootloops/swift,OscarSwanros/swift,rudkx/swift,austinzheng/swift,felix91gr/swift,brentdax/swift,lorentey/swift,karwa/swift,natecook1000/swift,nathawes/sw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