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

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

Pushed #145755 to revise the release note with information about numeric_conversion specifically.  If the underlying issue with boost::mpl::integral_c isn't actually impacting anyone, I won't worry about it.

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