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

[DWARF5][DebugInfo]: 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. Before this return type of the member function is deduced and
stored in the DIE.
This patch includes llvm 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