[clang] [clang] Turn -Wenum-constexpr-conversion into a hard error (PR #67170)

[Shafik Yaghmour via cfe-commits]

shafik wrote:

This change is a lot more involved, this is on my list of things to do. See the discussion here: https://reviews.llvm.org/D150226



https://github.com/llvm/llvm-project/pull/67170