DebugInfo: Further fix/improvements to r189494 (and LLVM r189495).

Selfhosting was crashing with the same type of problem but involving
template specializations.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@189530 91177308-0d34-0410-b5e6-96231b3b80d8