[clang] [clang][ast][objc] Implement getNameForDiagnostic for ObjCMethodDecl and ObjCPropertyDecl. (PR #213030)

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dmaclach wrote:

> > My understanding is that improving diagnostic messages is encouraged, and maintenance of any downstream tooling depending on them for regex parsing is up to the developer of such tooling :)
> 
> You are right. The PR looks good.
> 
> I want to lear more about your motivation. What made you writing this patch?

[I am working on improving](https://github.com/llvm/llvm-project/pulls?q=+is%3Apr+author%3Admaclach+created%3A%3E2026-05-01+) [include-cleaner](https://clangd.llvm.org/design/include-cleaner) for Objective-C.

The error messages I was seeing (ex: Header missing for 'NSObject::init') displeased me 😄 

https://github.com/llvm/llvm-project/pull/213030