[clang] [clang][diagnostics] Refactor "warn_doc_api_container_decl_mismatch" to use enum_select (PR #146433)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> Can we merge this with private email address?

Community policy is to have public email addresses: https://llvm.org/docs/DeveloperPolicy.html#email-addresses

https://github.com/llvm/llvm-project/pull/146433