Fix linkage related crash.

This test was exactly the opposite of what it should be. We should check if
there old decl has linkage (where it makes sense) and if the new decl has
the extern keyword.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@178735 91177308-0d34-0410-b5e6-96231b3b80d8
Add tests that build modules for our builtin headers, and fix two buglets exposed by doing so.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@178736 91177308-0d34-0410-b5e6-96231b3b80d8