[clang] [clang] CTAD: implement the missing IsDeducible constraint for alias templates (PR #89358)

[via cfe-commits]

https://github.com/cor3ntin approved this pull request.

I'll approve that modulo nit but i think we want

- An issue to keep track of the duplication in `FinishTemplateArgumentDeduction`
- A follow up PR to improve diagnostics


Thanks!

https://github.com/llvm/llvm-project/pull/89358