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

[Haojian Wu via cfe-commits]

hokein wrote:

I started a discussion on discourse: https://discourse.llvm.org/t/a-temporary-flag-for-guarding-access-to-an-in-development-c-20-feature-in-clang/76597.

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