[clang] [Sema] Suggest missing format attributes (PR #166738)

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https://github.com/apple-fcloutier approved this pull request.

Thanks, I think we have everything down now.

> I am not a fan of the diagnostic message for blocks, so let me know if you have a suggestion.

Format attributes on blocks are messy because they're declaration attributes rather than type attributes. I don't think that we need to spend more time on it than you already have.

I have 3 last small requests. I'm confident I won't need to review the changes again, so I'm approving early:

* Including "clang/Basic/CharInfo.h" is no longer necessary (I think).
* We need to pass a real source location to `isIgnored(..., SourceLocation())` to support diagnostic control through pragmas.
* We should change the test file names from attr-format-* to format-attr-* so that they are alphabetically grouped with the other format attribute tests.

https://github.com/llvm/llvm-project/pull/166738