Use the counted_by attribute of pointers in array bound checker.

Use the counted_by attribute of pointers in array bound checker.

Current array bound checker only instruments ARRAY_REF, and the INDEX
information is the 2nd operand of the ARRAY_REF.

When extending the array bound checker to pointer references with
counted_by attributes, the hardest part is to get the INDEX of the
corresponding array ref from the offset computation expression of
the pointer ref.  I.e.

Given an OFFSET expression, and the ELEMENT_SIZE,
get the index expression from the OFFSET.
For example:
  OFFSET:
   ((long unsigned int) m * (long unsigned int) SAVE_EXPR <n>) * 4
  ELEMENT_SIZE:
   (sizetype) SAVE_EXPR <n> * 4
get the index as (long unsigned int) m.

gcc/c-family/ChangeLog:

	* c-gimplify.cc (is_address_with_access_with_size): New function.
	(ubsan_walk_array_refs_r): Instrument an INDIRECT_REF whose base
	address is .ACCESS_WITH_SIZE or an address computation whose base
	address is .ACCESS_WITH_SIZE.
	* c-ubsan.cc (ubsan_instrument_bounds_pointer_address): New function.
	(struct factor_t): New structure.
	(get_factors_from_mul_expr): New function.
	(get_index_from_offset): New function.
	(get_index_from_pointer_addr_expr): New function.
	(is_instrumentable_pointer_array_address): New function.
	(ubsan_array_ref_instrumented_p): Change prototype.
	Handle MEM_REF in addtional to ARRAY_REF.
	(ubsan_maybe_instrument_array_ref): Handle MEM_REF in addtional
	to ARRAY_REF.

gcc/testsuite/ChangeLog:

	* gcc.dg/ubsan/pointer-counted-by-bounds-2.c: New test.
	* gcc.dg/ubsan/pointer-counted-by-bounds-3.c: New test.
	* gcc.dg/ubsan/pointer-counted-by-bounds-4.c: New test.
	* gcc.dg/ubsan/pointer-counted-by-bounds-5.c: New test.
	* gcc.dg/ubsan/pointer-counted-by-bounds.c: New test.