Sema: extract a check for `isCFError` (NFC)

Extract a simple check to check if a `RecordDecl` is a `CFError` Decl.
This is a simple refactoring to prepare for an upcoming change.  NFC.

Patch is extracted from
https://github.com/llvm/llvm-project-staging/commit/8afaf3aad2af43cfedca7a24cd817848c4e95c0c.