[clang] [clang] Track source deduction guide for alias template deduction guides (PR #123875)

[Erich Keane via cfe-commits]

https://github.com/erichkeane approved this pull request.

I think we're best-effort on the tests by now...  I don't see how we can do any better and still get value out of them.

https://github.com/llvm/llvm-project/pull/123875