[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.

Author: nate-chandler