[clang] Revert [Clang] prevent errors for deduction guides using deduced type aliases (PR #118165)

[Haojian Wu via cfe-commits]

https://github.com/hokein commented:

The change looks good to me (please wait for @erichkeane  review before landing). I’d suggest adding a brief rationale in the commit message instead of just including links, to make it more self-contained.

https://github.com/llvm/llvm-project/pull/118165