[clang] [clang][diagnostics] Refactor constexpr diagnostics to use enum_select (PR #179233)

[Shafik Yaghmour via cfe-commits]

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

This is definitely not implemented correctly. Please see other enum_select PRs from the past, understand how they are implemented and fix this PR to follow them.

You can not rely on a coding LLM to get these things correctly and if you don't understand why the changes are incorrect then this is an extractive PR, please see the AI policy for more details: https://llvm.org/docs/AIToolPolicy.html

https://github.com/llvm/llvm-project/pull/179233