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

[Haojian Wu via cfe-commits]

hokein 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. 

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