This patch refines the predicate that implements rule in RM 9.4 (11.9/2)

Compiling b94.ads must yield:

   b94.ads:11:17: illegal overriding of subprogram inherited from interface
   b94.ads:11:17: first formal of "N" declared at line 8 must be of mode
     "out", "in out" or access-to-variable

----
package B94 is

   type Prot2_Int is protected interface;
   procedure  J (PI : in     Prot2_Int; N : in Integer)  is null;
   procedure  K (PI : in out Prot2_Int; N : in Integer)  is null;
   procedure  L (PI :    out Prot2_Int; N : in Integer)  is null;
   procedure  M (PI : access Prot2_Int; N : in Integer)  is null;
   procedure  N (PI : access constant Prot2_Int; N : in Integer)  is null;

   protected type Protected_2 is new Prot2_Int with
      procedure N (N : in Integer);                   -- ERROR: {7;1}
   end Protected_2;

end B94;

2019-08-19  Ed Schonberg  <schonberg@adacore.com>

gcc/ada/

	* sem_ch6.adb (Check_Synchronized_Overriding): Complete
	predicate that applies legality check in 9.4 (11.9/2): if an
	inherited subprogram is implemented by a protected procedure or
	entry, its first paarameter must be out, in_out or
	access_to_varible.

From-SVN: r274655