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

Summary:
This change adds DIFlagNonTrivial to forward declarations of
DICompositeType. It adds the flag to nontrivial types and types with
unknown triviality.

It fixes adding the "CxxReturnUdt" flag to functions inconsistently,
since it is added based on whether the return type is marked NonTrivial, and
that changes if the return type was a forward declaration.

continues the discussion at https://reviews.llvm.org/D75215

Bug: https://bugs.llvm.org/show_bug.cgi?id=44785

Reviewers: rnk, dblaikie, aprantl

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D77436