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

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cor3ntin wrote:

> Regarding the __is_deducible type trait, GCC also provides one, but it was hidden from users and only used for internal CTAD implementation. I'm not sure if we should follow the same strategy in clang, ideas?

I have mixed feeling. What do you think @AaronBallman ?

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