Adopt llvm::function_ref in availability checking.
Adopt llvm::function_ref in availability checking.

The function is a callback parameter that does not escape, therefore
function_ref is the right choice.

No functionality change.

karwa/swift,karwa/swift,hooman/swift,Jnosh/swift,arvedviehweger/swift,tjw/swift,codestergit/swift,milseman/swift,stephentyrone/swift,Jnosh/swift,tjw/swift,swiftix/swift,xedin/swift,JaSpa/swift,austinzheng/swift,devincoughlin/swift,hooman/swift,hughbe/swift,danielmartin/swift,nathawes/swift,roambotics/swift,stephentyrone/swift,aschwaighofer/swift,harlanhaskins/swift,huonw/swift,lorentey/swift,benlangmuir/swift,milseman/swift,ahoppen/swift,tardieu/swift,practicalswift/swift,devincoughlin/swift,jmgc/swift,bitjammer/swift,gribozavr/swift,tinysun212/swift-windows,JaSpa/swift,JGiola/swift,zisko/swift,amraboelela/swift,bitjammer/swift,practicalswift/swift,jopamer/swift,natecook1000/swift,shahmishal/swift,deyton/swift,parkera/swift,hughbe/swift,Jnosh/swift,alblue/swift,frootloops/swift,zisko/swift,calebd/swift,rudkx/swift,harlanhaskins/swift,xedin/swift,hooman/swift,atrick/swift,return/swift,gribozavr/swift,gregomni/swift,atrick/swift,tinysun212/swift-windows,parkera/swift,jmgc/swift,sschiau/swift,tkremenek/swift,devincoughlin/swift,frootloops/swift,Jnosh/swift,tinysun212/swift-windows,bitjammer/swift,ahoppen/swift,CodaFi/swift,swiftix/swift,natecook1000/swift,felix91gr/swift,tkremenek/swift,shajrawi/swift,benlangmuir/swift,gregomni/swift,stephentyrone/swift,austinzheng/swift,jopamer/swift,parkera/swift,tinysun212/swift-windows,airspeedswift/swift,xwu/swift,djwbrown/swift,JGiola/swift,atrick/swift,bitjammer/swift,Jnosh/swift,airspeedswift/swift,lorentey/swift,huonw/swift,allevato/swift,djwbrown/swift,bitjammer/swift,uasys/swift,practicalswift/swift,manavgabhawala/swift,alblue/swift,frootloops/swift,hooman/swift,brentdax/swift,parkera/swift,bitjammer/swift,aschwaighofer/swift,frootloops/swift,zisko/swift,xwu/swift,CodaFi/swift,karwa/swift,xedin/swift,rudkx/swift,JGiola/swift,aschwaighofer/swift,JaSpa/swift,jopamer/swift,karwa/swift,hooman/swift,sschiau/swift,hughbe/swift,apple/swift,calebd/swift,xedin/swift,jtbandes/swift,milseman/swift,OscarSwanros/swift,CodaFi/swift,lorentey/swift,hughbe/swift,danielmartin/swift,hughbe/swift,huonw/swift,glessard/swift,jopamer/swift,devincoughlin/swift,gregomni/swift,jopamer/swift,gribozavr/swift,atrick/swift,harlanhaskins/swift,jtbandes/swift,alblue/swift,nathawes/swift,jtbandes/swift,aschwaighofer/swift,calebd/swift,shahmishal/swift,OscarSwanros/swift,codestergit/swift,benlangmuir/swift,hooman/swift,tkremenek/swift,harlanhaskins/swift,JGiola/swift,danielmartin/swift,lorentey/swift,sschiau/swift,swiftix/swift,calebd/swift,austinzheng/swift,shajrawi/swift,devincoughlin/swift,codestergit/swift,huonw/swift,deyton/swift,amraboelela/swift,natecook1000/swift,bitjammer/swift,tkremenek/swift,tjw/swift,xedin/swift,jckarter/swift,natecook1000/swift,rudkx/swift,uasys/swift,danielmartin/swift,natecook1000/swift,huonw/swift,karwa/swift,airspeedswift/swift,glessard/swift,brentdax/swift,tkremenek/swift,gribozavr/swift,alblue/swift,gregomni/swift,felix91gr/swift,Jnosh/swift,roambotics/swift,xwu/swift,OscarSwanros/swift,felix91gr/swift,shajrawi/swift,codestergit/swift,manavgabhawala/swift,djwbrown/swift,felix91gr/swift,jckarter/swift,alblue/swift,jckarter/swift,JaSpa/swift,sschiau/swift,allevato/swift,harlanhaskins/swift,parkera/swift,gribozavr/swift,swiftix/swift,devincoughlin/swift,gribozavr/swift,parkera/swift,CodaFi/swift,shahmishal/swift,shajrawi/swift,felix91gr/swift,frootloops/swift,nathawes/swift,austinzheng/swift,uasys/swift,manavgabhawala/swift,manavgabhawala/swift,glessard/swift,codestergit/swift,allevato/swift,allevato/swift,natecook1000/swift,return/swift,brentdax/swift,nathawes/swift,practicalswift/swift,jmgc/swift,JaSpa/swift,JaSpa/swift,calebd/swift,OscarSwanros/swift,swiftix/swift,glessard/swift,tjw/swift,stephentyrone/swift,parkera/swift,return/swift,shajrawi/swift,xwu/swift,austinzheng/swift,tardieu/swift,sschiau/swift,CodaFi/swift,practicalswift/swift,roambotics/swift,djwbrown/swift,tinysun212/swift-windows,arvedviehweger/swift,apple/swift,manavgabhawala/swift,nathawes/swift,stephentyrone/swift,harlanhaskins/swift,airspeedswift/swift,brentdax/swift,amraboelela/swift,codestergit/swift,felix91gr/swift,brentdax/swift,parkera/swift,karwa/swift,tjw/swift,jckarter/swift,glessard/swift,roambotics/swift,frootloops/swift,karwa/swift,uasys/swift,practicalswift/swift,allevato/swift,practicalswift/swift,shajrawi/swift,CodaFi/swift,hughbe/swift,uasys/swift,rudkx/swift,aschwaighofer/swift,deyton/swift,tardieu/swift,ahoppen/swift,allevato/swift,practicalswift/swift,sschiau/swift,sschiau/swift,milseman/swift,codestergit/swift,natecook1000/swift,aschwaighofer/swift,manavgabhawala/swift,glessard/swift,amraboelela/swift,swiftix/swift,lorentey/swift,shahmishal/swift,tinysun212/swift-windows,return/swift,airspeedswift/swift,alblue/swift,shahmishal/swift,tkremenek/swift,xedin/swift,tkremenek/swift,tardieu/swift,gribozavr/swift,deyton/swift,gregomni/swift,atrick/swift,hughbe/swift,arvedviehweger/swift,OscarSwanros/swift,rudkx/swift,tardieu/swift,arvedviehweger/swift,tinysun212/swift-windows,xedin/swi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