[Clang] Implement function attribute nouwtable

[Clang] Implement function attribute nouwtable

To have finer control of IR uwtable attribute generation. For target code generation,
IR nounwind and uwtable may have some interaction. However, for frontend, there are
no semantic interactions so the this new `nouwtable` is marked "SimpleHandler = 1".

Differential Revision: https://reviews.llvm.org/D132592