Add a comment about lexical lifetimes to CompilerInvocation.
Add a comment about lexical lifetimes to CompilerInvocation.

Otherwise, it's hard for me to tell who's in charge of setting the
default.

apple/swift,JGiola/swift,apple/swift,JGiola/swift,gregomni/swift,rudkx/swift,JGiola/swift,apple/swift,rudkx/swift,glessard/swift,atrick/swift,rudkx/swift,glessard/swift,atrick/swift,rudkx/swift,roambotics/swift,gregomni/swift,apple/swift,roambotics/swift,JGiola/swift,ahoppen/swift,apple/swift,roambotics/swift,benlangmuir/swift,gregomni/swift,benlangmuir/swift,ahoppen/swift,JGiola/swift,gregomni/swift,atrick/swift,benlangmuir/swift,ahoppen/swift,benlangmuir/swift,glessard/swift,atrick/swift,benlangmuir/swift,benlangmuir/swift,atrick/swift,rudkx/swift,JGiola/swift,glessard/swift,ahoppen/swift,apple/swift,gregomni/swift,rudkx/swift,atrick/swift,ahoppen/swift,roambotics/swift,ahoppen/swift,gregomni/swift,glessard/swift,roambotics/swift,roambotics/swift,glessard/swift