[Ada] Allow constants of access type in Global contracts

[Ada] Allow constants of access type in Global contracts

Now that SPARK supports access types, global constants of access type
may appear as outputs of a subprogram, with the meaning that the
underlying memory can be modified (see SPARK RM 3.10).

2019-09-19  Yannick Moy  <moy@adacore.com>

gcc/ada/

	* sem_prag.adb (Analyze_Global_In_Decl_Part): Do not issue an
	error when a constant of an access type is used as output in a
	Global contract.
	(Analyze_Depends_In_Decl_Part): Do not issue an error when a
	constant of an access type is used as output in a Depends
	contract.

gcc/testsuite/

	* gnat.dg/global2.adb, gnat.dg/global2.ads: New testcase.

From-SVN: r275947