Warning for mismatched declaration/definition with enum

[rth7680]

For a testcase such as

enum E { l = -1, z = 0, g = 1 };
int foo(void);
enum E foo(void) { return z; }

If the implementation type of enum E is not int, we will correctly emit an error (e.g. -fshort-enums).
It would be desirable to emit a warning in this case, because it is probably a mistake and definitely a portability error.