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

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cor3ntin 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) 

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