ada: Bug in Indefinite_Holders instance passed to formal package

Fix bug when an instance of Indefinite_Holders with a class-wide type is
passed as a generic formal package; Program_Error was raised when
dealing with the implicit "=" function.

The fix is to disable legality checks in formal packages when the
entity is an E_Subprogram_Body, because these are implicitly generated
for class-wide predefined functions when passed to generics.

gcc/ada/ChangeLog:

	* sem_ch12.adb (Check_Formal_Package_Instance):
	Do nothing in case of E_Subprogram_Body.

Author: bobduff

gcc/ada/sem_ch12.adb

@@ -7574,6 +7574,12 @@ package body Sem_Ch12 is
                     or else not Same_Instantiated_Function (E1, E2));
             end if;
 
+         --  No check is needed if this is the body of a subprogram that is
+         --  implicitly created in the case of class-wide predefined functions.
+
+         elsif Ekind (E1) = E_Subprogram_Body then
+            null;
+
          else
             raise Program_Error;
          end if;