[SILGenCleanup] Do not remove trivial lexical borrow scopes.
[SILGenCleanup] Do not remove trivial lexical borrow scopes.

This ensures that while in OSSA, we have in the IR information about the lexical
lifetime of let variables.

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