White-listing templated-scope friend decls is a good idea, but doing it

White-listing templated-scope friend decls is a good idea, but doing it
by marking the decl invalid isn't.  Make some steps towards supporting these
and then hastily shut them down at the last second by marking them as
unsupported.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@116661 91177308-0d34-0410-b5e6-96231b3b80d8