[DWARF5][clang]: Added support for DebugInfo generation for auto return type...

[DWARF5][clang]: Added support for DebugInfo generation for auto return type for C++ member functions.

Summary:
This patch will provide support for auto return type for the C++ member
functions.

This patch includes clang side implementation of this feature.

Patch by: Awanish Pandey <Awanish.Pandey@amd.com>

Reviewers: dblaikie, aprantl, shafik, alok, SouraVX, jini.susan.george
Reviewed by: dblaikie

Differential Revision: https://reviews.llvm.org/D70524