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

[Reid Kleckner via cfe-commits]

rnk wrote:

Internally, we discovered that this patch breaks Boost as used by MySQL. I don't have a full external reproducer at the moment and I don't know whether the code is valid or not, but I wanted to give a heads up that we may need to revisit this. This seems like the kind of incompatibility that would get reported during release testing later on.

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