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

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Sirraide wrote:

> Thanks @Sirraide for the review! I'd like to wait a couple of days before landing given that some folks are out for a wg21 meeting in these two weeks - in case they have other opinions.

Sure, but afaik you’ll have to wait until July most likely if you want to give other people time to look at this (unless Aaron or someone else who is not at the meeting can find the time to take a look at this). 

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