Function pointer implicitly convertible to any capability type in hybrid

[kevin-brodsky-arm]

Somehow, it seems that function pointers are implicitly convertible to any capability type in hybrid C:

int * __capability g(void)
{
    return g;
}

This yields no warning at all.

[arichardson]

If I recall correctly the silent conversion is a historical artifact of it being required in the CheriBSD hybrid kernel many years ago. However, also implicitly change the type rather than just the capability-ness is clearly a bug.

We should probably also remove the implicit conversion from function ptr -> function cap.