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

[via cfe-commits]

Sirraide wrote:

I will say, one worry that I do have is that this would end up issuing... a lot of notes for a single error:

![image](https://github.com/llvm/llvm-project/assets/74590115/3f517fb4-a00d-4093-b690-c2e2e1f49380)

C++ error messages already have a reputation of being rather long; I’m candidly doubtful as to whether this would help most users too much...

Perhaps an approach more similar to what we display for ambiguous cast paths would be better (https://godbolt.org/z/zT1rW7zcv):

![image](https://github.com/llvm/llvm-project/assets/74590115/8c4f5ba7-eec8-4c03-a026-7bfd23e01d6d)


i.e. list each implicit deduction guide we tried in a single note; I personally at leas think that using indentation and formatting a bit more rather than emitting a separate note for everything we might want to add to an error might go a long way in aiding legibility, but that’s just my opinion ;Þ

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