DebugInfo: Simplify/clarify propagation of typemembers between declaration and definition

Based on code review feedback from Eric Christopher (on r188739) and
Adrian Prantl (r188642).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@188829 91177308-0d34-0410-b5e6-96231b3b80d8