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

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cor3ntin wrote:

> This approach is doable technically, but it feels hacky and fragile. What if we emit an error (or warning) diagnostic and reject the code when we parse the __is_deducible type trait?

Why do you think it is fragile? I think a comment around `BTT_IsDeducible` would take care of that

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