[clang] Add test for CWG2213

[[https://wg21.link/p1787 | P1787]]: CWG2213 is resolved by allowing an elaborated-type-specifier to contain a simple-template-id without friend.
Wording: see changes to [dcl.type.elab]]/1.

The gist of the issue is that forward declaration of partial class template specialization was disallowed.

Reviewed By: #clang-language-wg, shafik

Differential Revision: https://reviews.llvm.org/D151032