[lto] Use -DLLVM_ENABLE_LTO to enable llvm lto instead of -DCMAKE_{C,CXX}_FLAGS
[lto] Use -DLLVM_ENABLE_LTO to enable llvm lto instead of -DCMAKE_{C,CXX}_FLAGS

This not only is the more canonical way to enable lto in LLVM but in addition
without this change, the swift compiler unittests will not build with LTO
resulting in funky LTO errors like rdar://25968091.

rdar://24717107

practicalswift/swift,shajrawi/swift,CodaFi/swift,shajrawi/swift,kperryua/swift,zisko/swift,tinysun212/swift-windows,gottesmm/swift,bitjammer/swift,amraboelela/swift,gmilos/swift,alblue/swift,jckarter/swift,swiftix/swift,codestergit/swift,natecook1000/swift,sschiau/swift,natecook1000/swift,OscarSwanros/swift,harlanhaskins/swift,milseman/swift,swiftix/swift,austinzheng/swift,airspeedswift/swift,russbishop/swift,xedin/swift,tinysun212/swift-windows,bitjammer/swift,russbishop/swift,parkera/swift,tjw/swift,harlanhaskins/swift,tjw/swift,CodaFi/swift,austinzheng/swift,deyton/swift,lorentey/swift,return/swift,xwu/swift,codestergit/swift,CodaFi/swift,rudkx/swift,OscarSwanros/swift,atrick/swift,calebd/swift,alblue/swift,sschiau/swift,arvedviehweger/swift,ben-ng/swift,Jnosh/swift,alblue/swift,jtbandes/swift,tardieu/swift,bitjammer/swift,kperryua/swift,shahmishal/swift,gmilos/swift,rudkx/swift,dreamsxin/swift,allevato/swift,milseman/swift,jckarter/swift,bitjammer/swift,jckarter/swift,devincoughlin/swift,tjw/swift,milseman/swift,nathawes/swift,rudkx/swift,roambotics/swift,tinysun212/swift-windows,natecook1000/swift,ben-ng/swift,karwa/swift,manavgabhawala/swift,uasys/swift,therealbnut/swift,jopamer/swift,nathawes/swift,rudkx/swift,danielmartin/swift,glessard/swift,nathawes/swift,calebd/swift,austinzheng/swift,shahmishal/swift,benlangmuir/swift,harlanhaskins/swift,calebd/swift,Jnosh/swift,calebd/swift,glessard/swift,swiftix/swift,hooman/swift,allevato/swift,therealbnut/swift,karwa/swift,huonw/swift,apple/swift,tjw/swift,xwu/swift,IngmarStein/swift,allevato/swift,hooman/swift,frootloops/swift,jopamer/swift,modocache/swift,ken0nek/swift,uasys/swift,jtbandes/swift,OscarSwanros/swift,benlangmuir/swift,alblue/swift,ben-ng/swift,hughbe/swift,brentdax/swift,sschiau/swift,harlanhaskins/swift,nathawes/swift,JGiola/swift,tkremenek/swift,ben-ng/swift,devincoughlin/swift,gmilos/swift,kperryua/swift,deyton/swift,Jnosh/swift,airspeedswift/swift,harlanhaskins/swift,tinysun212/swift-windows,OscarSwanros/swift,frootloops/swift,devincoughlin/swift,amraboelela/swift,airspeedswift/swift,brentdax/swift,swiftix/swift,lorentey/swift,apple/swift,amraboelela/swift,danielmartin/swift,codestergit/swift,aschwaighofer/swift,natecook1000/swift,felix91gr/swift,Jnosh/swift,brentdax/swift,gregomni/swift,manavgabhawala/swift,felix91gr/swift,glessard/swift,brentdax/swift,hooman/swift,amraboelela/swift,devincoughlin/swift,airspeedswift/swift,amraboelela/swift,devincoughlin/swift,karwa/swift,arvedviehweger/swift,stephentyrone/swift,jckarter/swift,CodaFi/swift,alblue/swift,natecook1000/swift,parkera/swift,deyton/swift,xwu/swift,xwu/swift,tkremenek/swift,sschiau/swift,benlangmuir/swift,apple/swift,harlanhaskins/swift,shahmishal/swift,kperryua/swift,austinzheng/swift,atrick/swift,uasys/swift,jopamer/swift,stephentyrone/swift,OscarSwanros/swift,zisko/swift,JGiola/swift,allevato/swift,jmgc/swift,JGiola/swift,tkremenek/swift,parkera/swift,modocache/swift,djwbrown/swift,ken0nek/swift,benlangmuir/swift,jmgc/swift,therealbnut/swift,deyton/swift,sschiau/swift,airspeedswift/swift,codestergit/swift,CodaFi/swift,shahmishal/swift,gmilos/swift,tinysun212/swift-windows,gregomni/swift,jopamer/swift,benlangmuir/swift,lorentey/swift,karwa/swift,gottesmm/swift,tardieu/swift,aschwaighofer/swift,roambotics/swift,uasys/swift,return/swift,therealbnut/swift,danielmartin/swift,jckarter/swift,JaSpa/swift,calebd/swift,modocache/swift,tardieu/swift,danielmartin/swift,hooman/swift,atrick/swift,felix91gr/swift,JaSpa/swift,jopamer/swift,sschiau/swift,frootloops/swift,zisko/swift,ahoppen/swift,aschwaighofer/swift,codestergit/swift,gregomni/swift,devincoughlin/swift,lorentey/swift,practicalswift/swift,ken0nek/swift,tkremenek/swift,zisko/swift,JGiola/swift,gribozavr/swift,shajrawi/swift,benlangmuir/swift,felix91gr/swift,jmgc/swift,russbishop/swift,amraboelela/swift,huonw/swift,airspeedswift/swift,gmilos/swift,gribozavr/swift,return/swift,ahoppen/swift,return/swift,jtbandes/swift,devincoughlin/swift,kstaring/swift,tinysun212/swift-windows,parkera/swift,glessard/swift,hughbe/swift,roambotics/swift,jmgc/swift,stephentyrone/swift,danielmartin/swift,modocache/swift,modocache/swift,russbishop/swift,deyton/swift,brentdax/swift,xedin/swift,tardieu/swift,uasys/swift,milseman/swift,tardieu/swift,manavgabhawala/swift,austinzheng/swift,uasys/swift,atrick/swift,manavgabhawala/swift,dreamsxin/swift,hooman/swift,aschwaighofer/swift,gottesmm/swift,jtbandes/swift,lorentey/swift,return/swift,stephentyrone/swift,hughbe/swift,JaSpa/swift,therealbnut/swift,djwbrown/swift,practicalswift/swift,OscarSwanros/swift,zisko/swift,practicalswift/swift,arvedviehweger/swift,IngmarStein/swift,bitjammer/swift,djwbrown/swift,apple/swift,glessard/swift,felix91gr/swift,rudkx/swift,gribozavr/swift,uasys/swift,jtbandes/swift,deyton/swift,karwa/swift,djwbrown/swift,ken0nek/swift,swiftix/swift,allevato/swift,xedin/swift,manavgabhawala/swift,karwa/swift,kstaring/swift,natecook1000/swift,aschwaighofer/swift,ahoppen/swift,zisko/swift,lorentey/swift,calebd/swift,arvedviehweger/swift,tjw/swift,modocache/swift,arvedviehweger/swift,codestergit/swift,jtbandes/swift,tinysun212/swift-windows,arvedviehweger/swift,manavgabhawala/swift,xedin/swift,karwa/swift,jtbandes/swift,tardieu/swift,alblue/swift,swiftix/swift,shahmishal/swift,gmilos/swift,devincoughlin/swift,gribozavr/swift,gregomni/swift,apple/swift,jopamer/swift,ahoppen/swift,ken0nek/swift,JaSpa/swift,shahmishal/swift,austinzheng/swift,kstaring/swift,practicalswift/swift,allevato/swift,tkremenek/swift,return/swift,lorentey/swift,natecook1000/swift,hughbe/swift,aschwaighofer/swift,russbishop/swift,tkremenek/swift,jmgc/swift,milseman/swift,swiftix/swift,hughbe/swift,karwa/swift,IngmarStein/swift,xedin/swift,atrick/swift,parkera/swift,shahmishal/swift,aschwaighofer/swift,gottesmm/swift,gmilos/swift,manavgabhawala/swift,ben-ng/swift,arvedviehweger/swift,amraboelela/swift,parkera/swift,gribozavr/swift,jckarter/swift,roambotics/swift,zisko/swift,modocache/swift,brentdax/swift,djwbrown/swift,return/swift,kstaring/swift,gottesmm/swift,roambotics/swift,shajrawi/swift,lorentey/swift,IngmarStein/swift,ken0nek/swift,gribozavr/swift,hughbe/swift,milseman/swift,shajrawi/swift,tjw/swift,practicalswift/swift,felix91gr/swift,practicalswift/swift,jopamer/swift,Jnosh/swift,frootloops/swift,danielmartin/swift,therealbnut/swift,felix91gr/swift,bitjammer/swift,nathawes/swift,shajrawi/swift,stephentyrone/swift,huonw/swift,ben-ng/swift,allevato/swift,harlanhaskins/swift,Jnosh/swift,gottesmm/swift,calebd/swift,frootloops/swift,CodaFi/swift,kperryua/swift,gregomni/swift,shajrawi/swift,JGiola/swift,therealbnut/swift,stephentyrone/swift,parkera/swift,kstaring/swift,djwbrown/swift,IngmarStein/swift,apple/swift,gribozavr/swift,deyton/swift,kstaring/swift,xedin/swift,brentdax/swift,huonw/swift,practicalswift/swift,djwbrown/swift,tkremenek/swift,ben-ng/swift,kperryua/swift,hooman/swift,glessard/swift,huonw/swift,JaSpa/swift,ahoppen/swift,shajrawi/swift,airspeedswift/swift,sschiau/swift,ahoppen/swift,austinzheng/swift,roambotics/swift,jmgc/swift,xwu/swift,atrick/swift,codestergit/swift,hooman/swift,shahmishal/swift,JaSpa/swift,xwu/swift,gottesmm/swift,IngmarStein/swift,sschiau/swift,xedin/swift,russbishop/swift,tardieu/swift,russbishop/swift,danielmartin/swift,Jnosh/swift,frootloops/swift,tjw/swift,JaSpa/swift,bitjammer/swift,nathawes/swift,nathawes/swift,parkera/swift,JGiola/swift,jckarter/swift,huonw/swift,kperryua/swift,IngmarStein/swift,hughbe/swift,ken0nek/swift,jmgc/swift,stephentyrone/swift,gregomni/swift,huonw/swift,milseman/swift,CodaFi/swift,rudkx/swift,gribozavr/swift,frootloops/swift,alblue/swift,xwu/swift,kstaring/swift,xedin/swift,OscarSwanros/swift