Pointer subtraction of differently-qualified pointers should not error

[ehaas]

The following errors with error: incompatible pointer types ('const char *' and 'char *')

#include <stddef.h>
ptrdiff_t foo(void) {
  char c[2] = {0, 0};
  const char *p  = &c[1];
  return p - &c[0];
}