Test DW_TAG_namespace support in the backend

This is the backend portion of a Clang test case
(clang/test/CodeGenCXX/debug-info-namespace.cpp) that was roughly/coarsely
testing LLVM.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@177487 91177308-0d34-0410-b5e6-96231b3b80d8