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

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cor3ntin wrote:

> 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.

I think we should have a custom diag for that anyway
"is not deducible from" is better than " `__is_deducible<>` is false"


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