[DebugInfo] Fix DWARF expressions for __block vars that are not on the heap

`__block` variables used to be always stored on the head instead of stack.
D51564 allowed `__block` variables to the stored on the stack like normal
variablesif they not captured by any escaping block, but the debug-info
generation code wasn't made aware of it so we still unconditionally emit DWARF
expressions pointing to the heap.

This patch makes CGDebugInfo use the `EscapingByref` introduced in D51564 that
tracks whether the `__block` variable is actually on the heap. If it's stored on
the stack instead we just use the debug info we would generate for normal
variables instead.

Reviewed By: ahatanak, aprantl

Differential Revision: https://reviews.llvm.org/D99946