PR28423: Compare primary declaration contexts.

In certain cases (mostly coming from modules), Sema's idea of the StdNamespace
does not point to the first declaration of namespace std.

Patch by Cristina Cristescu!

Reviewed by Richard Smith.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@279371 91177308-0d34-0410-b5e6-96231b3b80d8