Enums types with fixed underlying type should compare equal to underlying type

[ehaas]

The following code should not produce any error diagnostics:

enum E {
    A,
};

enum E_signed: int {
    B,
};

enum E_unsigned: unsigned {
    C,
};

_Static_assert(!__builtin_types_compatible_p(enum E, enum E_signed), "");
_Static_assert(!__builtin_types_compatible_p(enum E, enum E_unsigned), "");

_Static_assert(__builtin_types_compatible_p(int, enum E_signed), "");
_Static_assert(!__builtin_types_compatible_p(unsigned, enum E_signed), "");

_Static_assert(!__builtin_types_compatible_p(int, enum E_unsigned), "");
_Static_assert(__builtin_types_compatible_p(unsigned, enum E_unsigned), "");