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

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> @AaronBallman If you try to compile the following c program after this patch
> 
> ```
> void free ();
> typedef enum {REG_EESCAPE} reg_errcode_t;
> typedef struct {int *stack;} compile_stack_type;
> reg_errcode_t byte_regex_compile () {
>   compile_stack_type compile_stack;
>   return (free (compile_stack.stack), REG_EESCAPE);                                                                                                                                
> }
> ```
> 
> cmd: `clang -Wimplicit-int-enum-cast -c pre.i`
> 
> It will give an incorrect diagnostic:
> 
> ```
> pre.i:6:37: warning: implicit conversion from 'int' to enumeration type 'reg_errcode_t' is invalid in C++ [-Wimplicit-int-enum-cast]
>     6 |   return (free (compile_stack.stack), REG_EESCAPE);                                                                                                                                
>       |   ~~~~~~  ~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~
> 1 warning generated.
> ```

https://github.com/llvm/llvm-project/pull/138752 should address this issue, thank you for bringing it up!

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