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

[Lei Wang via cfe-commits]

wlei-llvm 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.

+1, this also breaks our internal Boost which is on 1.77.0. I'm checking out the latest Boost to verify if there is already a fix. 

@rnk Do you know yet if it also breaks the latest Boost? 

I found some related fixes in Boost that may help: 
https://github.com/boostorg/numeric_conversion/commit/50a1eae942effb0a9b90724323ef8f2a67e7984a

https://github.com/percona/percona-server-mongodb/commit/8e1167e3389f8df77e8794dcb8d4dece9e5d674d

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