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

[Jordan Rupprecht via cfe-commits]

rupprecht 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://www.boost.io/calendar/ says this is scheduled for tomorrow. How soon after 1.86 landing do you plan to submit this?

No objections from us if you'd like to land this whenever you're ready. We have some breakage to deal with, but it's manageable. Thanks for giving the clang vendors list a heads up!

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