[clang][diagnostics] Refactor `warn_doc_function_method_decl_mismatch` to use enum_select (#181769)
Related: https://github.com/llvm/llvm-project/issues/123121
This patch refactors the `warn_doc_function_method_decl_mismatch`
diagnostic to use `enum_select` instead of `select`. This gets rid of
magic numbers in its caller and improves readability.