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

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

Summary:
This was reverted in e45fcfc3aa57bb237fd4fd694d0c257be66d5482 due to
libcxx build failure. This revision addresses that case.

Original commit message:
    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