AST: Fix __uuidof for template specializations

While we previously supported __uuidof applied to a template
specialization, we would only find the uuid attribute if it was applied
to the template argument.  We would erroneously ignore the uuid
attribute on the specialization itself.

This is required to parse Windows Runtime C++ Template Library headers.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@213016 91177308-0d34-0410-b5e6-96231b3b80d8