[Ada] Wrong code in array aggregates of Ada coextensions

[Ada] Wrong code in array aggregates of Ada coextensions

The compiler generates wrong code when an array aggregate with an others choice
whose expression has nested object allocations (ie. others => new R (new S)) is
used to initialize an array of access to discriminated types whose discriminant
is an access type.

2018-06-11  Javier Miranda  <miranda@adacore.com>

gcc/ada/

	* sinfo.ads (Is_Dynamic_Coextension): Adding documentation.
	(Is_Static_Coextension): Adding documentation.
	* sinfo.adb (Is_Dynamic_Coextension): Extending the assertion.
	(Is_Static_Coextension): Extending the assertion.
	* sem_util.adb (Mark_Allocator): Clear Is_Static_Coextension when
	setting flag Is_Dynamic_Coextension (and vice versa).

gcc/testsuite/

	* gnat.dg/aggr23.adb, gnat.dg/aggr23_q.adb, gnat.dg/aggr23_tt.ads: New
	testcase.

From-SVN: r261406