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

[Aaron Ballman via cfe-commits]

AaronBallman 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 ?

Personally, I do not like exposing type traits that aren't for use with the STL. One idea would be to remove the type trait from TokenKinds.def and instead manually add `BTT_IsDeducible` to: https://github.com/llvm/llvm-project/blob/c61f0a8e94004b05d9ec115d3bff8cff331b4491/clang/include/clang/Basic/TypeTraits.h#L21 along with a comment explaining that this is for internal use only rather than be exposed to users. (You'd have to see if there are other places using `TYPE_TRAIT_2` that might need adjustment as well.) Then we can remove the release note, error checking can become assertions, etc.

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