Don't use a global_ctors comdat for globals that aren't externally visible

In particular, if you have two identical templates in different TUs in
anonymous namespaces, we would use the same global_ctors comdat key for
both. As a result, only one would be run.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@219806 91177308-0d34-0410-b5e6-96231b3b80d8