[DebugInfo] Fix for adding "returns cxx udt" option to functions in CodeView.

Summary:
This change checks for the return type in the frontend and adds a flag
to the DISubroutineType to indicate that the option should be added in
CodeViewDebug.

Previously function types sometimes appeared twice in the PDB: once with
"returns cxx udt" and once without.
See https://bugs.llvm.org/show_bug.cgi?id=44785.

Reviewers: rnk, asmith

Subscribers: hiraditya, cfe-commits, llvm-commits

Tags: #clang, #llvm

Differential Revision: https://reviews.llvm.org/D75215