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

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cor3ntin wrote:

> I agree with you -- having a well-described diagnostic message is better and clearer. I'm happy to improve it once we settle on the final implementation approach (the current diagnostic because '__is_deducible(AFoo, Foo<int>)' evaluated to false seems okay to me. GCC also emits similar diagnostics).

Well, if we agree on that, the only thing to do for approach 3 is to deal with "anonymous" traits in ast dump and similar, which seems to be a fairly bounded effort!

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