AST: Add some assertions, NFC
AST: Add some assertions, NFC

Easier to debug if we crash when constructing an invalid Requirement
rather than later on when we try to do something with it.

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