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

[Haojian Wu via cfe-commits]

hokein wrote:

> > OK, I have hidden the `__is_deducible` trait as suggested. Please take another look.
> 
> Sorry, I think my suggestion was not clear enough. did you try to put `TYPE_TRAIT_2(/**/, IsDeducible, KEYCXX)` in `TokenKinds.def` (and then other headers should _NOT_ be modified)

Ah, I see. I haven't tried that, but one downside I can see is that it will make diagnostics for CTAD worse, we need to spell the type trait name in error messages when the `__is_deducible` trait fails.

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