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

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

Most likely, the user code is invalid.  I was hoping we would get away without hitting any bugs in major frameworks, but I'm not surprised we hit something.

If this is widespread, we can look into mitigations, but it's difficult to downgrade constant evaluation errors to warnings.  (Particularly if SFINAE is involved.)

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