PR16995: DebugInfo: Don't overwrite existing member lists when adding template arguments

With the added debug assertions this fix is covered by existing Clang
tests. (& found some other issues, also fixed)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@189495 91177308-0d34-0410-b5e6-96231b3b80d8