Fix false positive on anonymous namespaces in headers.

Summary:
Anynoumous namespaces inject a using directive into the AST to import
the names into the containing namespace.
We should not have them in headers, but there is another warning for
that.

Reviewers: djasper

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D8443

llvm-svn: 233087