[clang] [C] Add new -Wimplicit-int-enum-cast to -Wc++-compat (PR #137658)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

Ooof, this may not be easy to work around. In C, we're in a context where we know we have an implicit conversion. But we have no way of knowing that we would not have the implicit conversion in C++. We could probably handle comma expressions as a special case, but I suspect that will just move the issue to another place. Still investigating whether there's a good way to salvage this.

https://github.com/llvm/llvm-project/pull/137658