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

[Reid Kleckner via cfe-commits]

rnk wrote:

If we've had fixes available for years, I think backporting them would be the right call, but this seems like it might be a release-note-worthy change.

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