Revisit implementation of custom unwind resume;

Revisit implementation of custom unwind resume;
Rather than injecting a local `_Unwind_Resume` into the current translation unit,
just replace `resume` instruction with a direct call the the `eh_unwind_resume` lang item.
This is likely to be more robust in the face of future LLVM changes, and also allows us to delegate
work back to libgcc's `_Unwind_Resume`.