[Ada] Post warning on object size clause for subtype

[Ada] Post warning on object size clause for subtype

This ensures that a warning for an object size clause present on a subtype
is posted on the clause and not on a size clause present on the type.

2018-05-31  Eric Botcazou  <ebotcazou@adacore.com>

gcc/ada/

	* einfo.ads (Object_Size_Clause): Declare.
	* einfo.adb (Object_Size_Clause): New function.
	* gcc-interface/utils.c (maybe_pad_type): Test Has_Size_Clause before
	retrieving Size_Clause and post the warning on the object size clause
	if Has_Object_Size_Clause is true.

gcc/testsuite/

	* gnat.dg/size_clause1.adb: New testcase.

From-SVN: r260998