Add a comment to `canUnsafeCast` in SILType.cpp.
Add a comment to `canUnsafeCast` in SILType.cpp.

I added a FIXME so that no one confuses the current implementation with the
ABI's type layout rules. (The implementation was conservative out of paranoia.)

zisko/swift,roambotics/swift,JGiola/swift,hooman/swift,sschiau/swift,amraboelela/swift,zisko/swift,tjw/swift,rudkx/swift,xwu/swift,amraboelela/swift,stephentyrone/swift,xedin/swift,huonw/swift,CodaFi/swift,jmgc/swift,apple/swift,karwa/swift,danielmartin/swift,jopamer/swift,stephentyrone/swift,jopamer/swift,brentdax/swift,gribozavr/swift,amraboelela/swift,gribozavr/swift,xwu/swift,airspeedswift/swift,karwa/swift,shajrawi/swift,harlanhaskins/swift,natecook1000/swift,brentdax/swift,xedin/swift,CodaFi/swift,stephentyrone/swift,apple/swift,austinzheng/swift,devincoughlin/swift,apple/swift,brentdax/swift,roambotics/swift,allevato/swift,JGiola/swift,zisko/swift,natecook1000/swift,amraboelela/swift,glessard/swift,gregomni/swift,parkera/swift,danielmartin/swift,roambotics/swift,parkera/swift,JGiola/swift,allevato/swift,sschiau/swift,alblue/swift,airspeedswift/swift,practicalswift/swift,jopamer/swift,benlangmuir/swift,sschiau/swift,gregomni/swift,apple/swift,ahoppen/swift,allevato/swift,devincoughlin/swift,sschiau/swift,jmgc/swift,xwu/swift,tjw/swift,rudkx/swift,huonw/swift,JGiola/swift,nathawes/swift,aschwaighofer/swift,shahmishal/swift,natecook1000/swift,stephentyrone/swift,huonw/swift,nathawes/swift,airspeedswift/swift,tjw/swift,nathawes/swift,ahoppen/swift,hooman/swift,benlangmuir/swift,glessard/swift,lorentey/swift,sschiau/swift,lorentey/swift,jopamer/swift,jmgc/swift,brentdax/swift,allevato/swift,ahoppen/swift,CodaFi/swift,gribozavr/swift,apple/swift,zisko/swift,benlangmuir/swift,devincoughlin/swift,zisko/swift,alblue/swift,devincoughlin/swift,alblue/swift,atrick/swift,tjw/swift,roambotics/swift,tkremenek/swift,lorentey/swift,parkera/swift,gregomni/swift,tkremenek/swift,zisko/swift,brentdax/swift,jmgc/swift,shahmishal/swift,jopamer/swift,glessard/swift,rudkx/swift,danielmartin/swift,alblue/swift,amraboelela/swift,shajrawi/swift,gregomni/swift,atrick/swift,gregomni/swift,xedin/swift,CodaFi/swift,practicalswift/swift,practicalswift/swift,CodaFi/swift,austinzheng/swift,ahoppen/swift,karwa/swift,harlanhaskins/swift,xwu/swift,benlangmuir/swift,lorentey/swift,gribozavr/swift,danielmartin/swift,aschwaighofer/swift,roambotics/swift,nathawes/swift,airspeedswift/swift,gribozavr/swift,huonw/swift,shahmishal/swift,alblue/swift,glessard/swift,apple/swift,sschiau/swift,devincoughlin/swift,glessard/swift,lorentey/swift,shahmishal/swift,austinzheng/swift,gribozavr/swift,shajrawi/swift,airspeedswift/swift,xwu/swift,harlanhaskins/swift,shahmishal/swift,tkremenek/swift,jmgc/swift,jopamer/swift,allevato/swift,tjw/swift,nathawes/swift,jckarter/swift,stephentyrone/swift,xedin/swift,xedin/swift,danielmartin/swift,jckarter/swift,xedin/swift,tkremenek/swift,stephentyrone/swift,shahmishal/swift,aschwaighofer/swift,natecook1000/swift,practicalswift/swift,sschiau/swift,aschwaighofer/swift,jckarter/swift,parkera/swift,natecook1000/swift,harlanhaskins/swift,karwa/swift,xwu/swift,airspeedswift/swift,parkera/swift,rudkx/swift,parkera/swift,karwa/swift,xedin/swift,lorentey/swift,brentdax/swift,stephentyrone/swift,aschwaighofer/swift,practicalswift/swift,atrick/swift,hooman/swift,karwa/swift,jmgc/swift,karwa/swift,devincoughlin/swift,xedin/swift,glessard/swift,rudkx/swift,harlanhaskins/swift,danielmartin/swift,aschwaighofer/swift,atrick/swift,gregomni/swift,alblue/swift,huonw/swift,sschiau/swift,allevato/swift,airspeedswift/swift,harlanhaskins/swift,practicalswift/swift,shajrawi/swift,alblue/swift,austinzheng/swift,jopamer/swift,shajrawi/swift,atrick/swift,practicalswift/swift,huonw/swift,tjw/swift,amraboelela/swift,harlanhaskins/swift,natecook1000/swift,devincoughlin/swift,austinzheng/swift,shahmishal/swift,CodaFi/swift,huonw/swift,jckarter/swift,hooman/swift,zisko/swift,tkremenek/swift,tkremenek/swift,hooman/swift,rudkx/swift,jckarter/swift,devincoughlin/swift,roambotics/swift,parkera/swift,brentdax/swift,nathawes/swift,shajrawi/swift,natecook1000/swift,benlangmuir/swift,hooman/swift,jckarter/swift,jmgc/swift,nathawes/swift,tjw/swift,aschwaighofer/swift,xwu/swift,amraboelela/swift,shajrawi/swift,danielmartin/swift,allevato/swift,lorentey/swift,practicalswift/swift,shahmishal/swift,lorentey/swift,CodaFi/swift,gribozavr/swift,JGiola/swift,ahoppen/swift,gribozavr/swift,karwa/swift,austinzheng/swift,ahoppen/swift,parkera/swift,shajrawi/swift,jckarter/swift,atrick/swift,austinzheng/swift,JGiola/swift,tkremenek/swift,benlangmuir/swift,hooman/swift