[clang] [Clang] Clarify diagnostic notes for implicitly generated deduction guides (PR #96084)

[Younan Zhang via cfe-commits]

zyn0217 wrote:

Thank you folks for the review & suggestion. I plan to merge this PR next week in case @AaronBallman or @cor3ntin has objections.

https://github.com/llvm/llvm-project/pull/96084