[clang] [clang-tools-extra] [Clang][Sema] Clarify structured binding diagnostic when initializer is missing (PR #181803)

[Yanzuo Liu via cfe-commits]

zwuis wrote:

> @zwuis  While #181579 is very clearly undisclosed use of AI, I don't see any real signs of that in this PR?

Oh, I might overthink. He did follow review comments from AaronBallman but current changes in SemaDecl.cpp is actually NFC.

https://github.com/llvm/llvm-project/pull/181803