[clang] [clang] Refactor `IdentifierInfo::ObjcOrBuiltinID` (PR #71709)

[Vlad Serebrennikov via cfe-commits]

Endilll wrote:

> Oh, I didn't look into the identifier's system before. I took a while to look at the patch but I failed to understand it and I failed to find the relationships between this patch and header units...

Yeah, the part this PR touches in not the most straightforward one. Thank you for you time!

https://github.com/llvm/llvm-project/pull/71709