PR middle-end/92815 - spurious -Wstringop-overflow writing into a flexible array...

PR middle-end/92815 - spurious -Wstringop-overflow writing into a flexible array of an extern struct

gcc/ChangeLog:

	PR middle-end/92815
	* tree-object-size.c (decl_init_size): New function.
	(addr_object_size): Call it.
	* tree.h (last_field): Declare.
	(first_field): Add attribute nonnull.

gcc/testsuite/ChangeLog:

	PR middle-end/92815
	* gcc.dg/Warray-bounds-56.c: Remove xfails.
	* gcc.dg/builtin-object-size-20.c: New test.
	* gcc.dg/builtin-object-size-21.c: New test.