Like `?` or property access, `x!` can be borrowing, consuming, or mutated
through depending on the use site and the ownership of the base value.
Alter SILGen to emit `x!` as a borrowing operation when the result is only
used as a borrow. Fix the move-only checker not to treat the unreachable
branch as a dead path for values and try to destroy the value unnecessarily
and possibly out-of-order with cleanups on the value. Fixes rdar://127459955.
