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

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cor3ntin wrote:

> BTW, should we close #54051 with this PR? or should we keep it open until the feature is fully implemented? The `is_deducible` part is a major missing piece, I think we can close #54051 and open a separate issue to track the missing piece, WDYT?

Sounds good

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