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

[Haojian Wu via cfe-commits]

hokein 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)' 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!

I was worried about the potential need to handle these issues in various places. However, thinking more about it today, I think we can address them locally in the `getTraitSpelling`, which seems like a more favorable approach.

I've now updated the patch to implement approach 3. Please take a look. Note that I haven't addressed the diagnostic improvement part yet; I plan to handle that in a separate follow-up patch.



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