[clang] [clang] Turn -Wenum-constexpr-conversion into a hard error (PR #102364)

[Jordan Rupprecht via cfe-commits]

rupprecht wrote:

The last time I followed this attempt was https://reviews.llvm.org/D150226, where there was objection because it was ignored in system headers, so effectively impossible to discover how widespread this is. When did this become unignored in system headers?

I remember fixing almost all the issues in our codebase for non-system headers a long time ago, so I don't think we'll have much objection to this. But maybe I should wait for those results to finish before committing to that :)

https://github.com/llvm/llvm-project/pull/102364