[Clang] Correct handling of negative and out-of-bounds indices (#71877)

GCC returns 0 for a negative index on an array in a structure. It also
returns 0 for an array index that goes beyond the extent of the array.
In addition. a pointer to a struct field returns that field's size, not
the size of it plus the rest of the struct, unless it's the first field
in the struct.

  struct s {
    int count;
    char dummy;
    int array[] __attribute((counted_by(count)));
  };

  struct s *p = malloc(...);

  p->count = 10;

A __bdos on the elements of p return:

  __bdos(p, 0) == 30
  __bdos(p->array, 0) == 10
  __bdos(&p->array[0], 0) == 10
  __bdos(&p->array[-1], 0) == 0
  __bdos(&p->array[42], 0) == 0

Also perform some refactoring, putting the "counted_by" calculations in
their own function.