Const-correctness broken for references to pointers.

[Hyman Rosen]

The compiler allows a reference to a pointer to constant to be initialized
by a pointer to non-constant. This allows const data to be changed, without
a peep from the compiler. Here's the test case:

/*----------------------------------------*/
extern "C" int printf(const char *, ...);
int main(int, char **)
{
	volatile const int c = 42;
	volatile int *p;
	volatile const int *&r = p;
	r = &c;
	*p = 0;
	if (c != 42)
		printf("FAILED!\n");
}
/*----------------------------------------*/