Enhance debug info namespace test to check for context/scope reference

The #line directive is mostly for backend testing (keeping these files matching
should simplify maintenance somewhat) though the corresponding backend test
improvement/update doesn't verify the file information directly just yet.
Coming in a later iteration.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@177559 91177308-0d34-0410-b5e6-96231b3b80d8