[clang] [CodeGen][CIR] Split out Backend Diags to utils, impl CIR error-attr (PR #225966)

[Andy Kaylor via cfe-commits]

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andykaylor wrote:

Claude says this include, along with DiagnosticPrinter.h, Demangle.h, and Hashing.h are now unused.

https://github.com/llvm/llvm-project/pull/225966