PR14606: Debug Info for namespace aliases/DW_TAG_imported_module

This resolves the last of the PR14606 failures in the GDB 7.5 test
suite. (but there are still unresolved issues in the imported_decl case
- we need to implement optional/lazy decls for functions & variables
like we already do for types)

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@182329 91177308-0d34-0410-b5e6-96231b3b80d8