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

[Shafik Yaghmour via cfe-commits]

https://github.com/shafik commented:

In order to aid the discussion on the temporary flag, I think it would be helpful to have a full set of tests that show what elements won't work right with the current set of changes.

I think this will help us gauge how impactful partial support would be and maybe help guide the discussion on alternatives.

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