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

[Carlos Galvez via cfe-commits]

carlosgalvezp wrote:

Since boost/mpl is at the core of issues and many projects depend directly or transitively on it, I think it might be good to wait until version 1.86 is released, so people can bump to a release version instead of trunk. 

It should be around the corner, [AFAICS](https://www.boost.org/users/history/) they've been releasing around this date regularly in the past years.

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