tree-optimization/120780: Support object size for containing objects

tree-optimization/120780: Support object size for containing objects

MEM_REF cast of a subobject to its containing object has negative
offsets, which objsz sees as an invalid access.  Support this use case
by peeking into the structure to validate that the containing object
indeed contains a type of the subobject at that offset and if present,
adjust the wholesize for the object to allow the negative offset.

gcc/ChangeLog:

	PR tree-optimization/120780
	* tree-object-size.cc (inner_at_offset,
	get_wholesize_for_memref): New functions.
	(addr_object_size): Call get_wholesize_for_memref.

gcc/testsuite/ChangeLog:

	PR tree-optimization/120780
	* gcc.dg/builtin-dynamic-object-size-pr120780.c: New test case.

Signed-off-by: Siddhesh Poyarekar <siddhesh@gotplt.org>