Implement merging of namespace-scope declarations across modules, so

Implement merging of namespace-scope declarations across modules, so
that we can merge, for example, two occurrences of

  namespace N { void f(); }

in two disjoint modules.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@147780 91177308-0d34-0410-b5e6-96231b3b80d8