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