[clang] [Clang] prevent errors for deduction guides using deduced type aliases (PR #117450)

[Erich Keane via cfe-commits]

erichkeane wrote:

Hmmm... See the comment on the issue. I'll have to look more closely, but this very well might be the incorrect change. This perhaps needs a revert.

https://github.com/llvm/llvm-project/pull/117450