Debug info - generalize namespace test to not depend on a DW_TAG_file_type entry

This isn't necessary & with the next change to LLVM the DW_TAG_file_type entry
won't be emitted at all - only the raw filename/directory pair, so match on
that directly instead.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@177609 91177308-0d34-0410-b5e6-96231b3b80d8