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

[via cfe-commits]

asmok-g wrote:

@AaronBallman I met another case, giving the same error even after the fix. Sorry I only noticed it now:
```
const char *p;
const char *pend;
...
return (free (compile_stack.stack), p == pend ? REG_EBRACE : REG_BADBR));
```

Both REG_EBRACE, REG_BADBR are `reg_errcode_t `

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