[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?
> 
> [e7a6171](https://github.com/llvm/llvm-project/commit/e7a6171c4c0486aae051543e202d74c04178cfac)

Thanks!

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