The patch gives an error message on "for T'Object_Size use 0;".

2019-08-20  Bob Duff  <duff@adacore.com>

gcc/ada/

	* sem_ch13.adb (Object_Size): Give an error for zero. It really
	rubs me the wrong way that we don't honor "for T'Object_Size use
	0;", but it's not important enough to fix. In any case, if we're
	not going to obey the clause, we should give an error.

gcc/testsuite/

	* gnat.dg/object_size1.adb: New testcase.

From-SVN: r274722