[clang] [clang] Check constexpr int->enum conversions consistently. (PR #143034)

[Erich Keane via cfe-commits]

erichkeane wrote:

@efriedma-quic : If you haven't already, please add a release note that is pretty descriptive of the error/issue here, so that people can find it in the future.  Additionally, a discourse post to the 'potentially-breaking' tag would be a VERY useful place to socialize this.

https://github.com/llvm/llvm-project/pull/143034