[clang] [clang] WIP: Implement CTAD for type alias template. (PR #77890)

[Haojian Wu via cfe-commits]

hokein wrote:

> Note that Clang 18 is going to branch soon, so this PR is going to target Clang 19.

Yes, this patch will not be ready before the 18 release cut, so targeting clang 19 sounds like a good plan.

As you may notice, the implementation is incomplete, and is likely far from perfect, I can see two alternatives to proceed:
1) we keep improving it here, and landing it when it is in good/perfect shape;
2) find a way to land a relatively-good initial version, and continue to develop it incrementally;

Re 1), it is hard to predict the timeline, I'd like to finish it soon, but I also have other tasks with higher priority.
2) seems like a good option, we can hide this experimental feature with a new `-cc1` flag, when this feature is ready, we enable it by default. It is probably not a common practice for a narrow feature like this. 

Any thoughts?  @cor3ntin, @shafik, @AaronBallman 


https://github.com/llvm/llvm-project/pull/77890