[Mem2Reg] Lexical allocs create lexical borrows.
SILGen turns vars into alloc_boxes. When possible, AllocBoxToStack
turns those into alloc_stacks. In order to preserve the lexical
lifetime of those vars, the alloc_stacks are annotated with the
[lexical] attribute. When Mem2Reg runs, it promotes the loads from
and stores into those alloc_stacks to uses of registers. In order to
preserve the lexical lifetime during that transformation, lexical borrow
scopes must be introduces that encode the same lifetime as the
alloc_stacks did. Here, that is done.
