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

[Erich Keane via cfe-commits]

erichkeane wrote:

> @cor3ntin, @erichkeane, thanks for all review comments. I'm not sure you have reviewed the whole patch. My feeling is that the core part (`DeclareImplicitDeductionGuidesForTypeAlias`) may not have received sufficient reviews. It would be great if you can take a closer look at this part.

I've looked through it a few times(including again now) and didn't see anything that stuck out as wrong to me.  I don't believe I can review it any more thoroughly.

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