[clang] [clang][diagnostics] Refactor `warn_doc_function_method_decl_mismatch` to use enum_select (PR #181769)

[Yanzuo Liu via cfe-commits]

https://github.com/zwuis requested changes to this pull request.

> PR description seems to mention the wrong diagnostic.

https://github.com/llvm/llvm-project/pull/181769