[clang] [clang] CTAD: fix the aggregate deduction guide for alias templates. (PR #90894)

[Erich Keane via cfe-commits]

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

I think I'm ok with this, please give others a day or two to sign off as well.

https://github.com/llvm/llvm-project/pull/90894