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

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cor3ntin wrote:

I'm thrilled to see work in this area. Thanks!

Note that Clang 18 is going to branch soon, so this PR is going to target Clang 19.
However, because it took a while to stabilize other CTAD related works, i think it might be worth it to try to complete that work early in the Clang 19 cycle, so that we have a few months to weed out the bugs.

I do believe this is the last unimplemented features for our C++20 support (outside of bugs and modules)

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