[Ada] Illegal protected calls in inherited pre/postconditions

[Pierre-Marie de Rodat]

AI12-0166 specifies that it is illegal for a pre/postcondition of a
protected operation to contain an internal call to a protected function.
This patch completes the implementation of this rule in the case the
condition is inherited from a classwide condition of an abstract operation
of an interface type.

Compiling inheritpo.adb must yield:

   inheritpo.ads:9:04: instantiation error at line 6
   inheritpo.ads:9:04: internal call to "F" cannot appear
      in inherited precondition of protected operation "P"
   inheritpo.ads:9:04: instantiation error at line 7
   inheritpo.ads:9:04: internal call to "F" cannot appear
      in inherited precondition of protected operation "P"

--
package InheritPO is

   type T is limited interface;
   function F (X : T) return Boolean is abstract;
   procedure P (X : in out T) is abstract with
     Pre'Class  => X.F,
     Post'Class => X.F;

   protected type PT is new T with
     overriding function F return Boolean;
     overriding procedure P;
   end PT;

end InheritPO;
----
package body InheritPO is
   protected body PT is
     function F return Boolean is begin return True; end;
     procedure P is begin null; end;
   end PT;
end InheritPO;

Tested on x86_64-pc-linux-gnu, committed on trunk

2018-01-11  Ed Schonberg  <schonberg@adacore.com>

gcc/ada/

	* sem_ch3.adb (Add_Internal_Interface_Entities): When checking the
	legality of an inherited operation that may require overriding, ignore
	primitive_wrappers that correspond to explicit operations that override
	an interface primitive.
	* exp_util.adb (Build_Class_Wide_Expression, Replace_Entity): If the
	operation to which the class-wide expression applies is a protected op.
	with a primitive_wrapper, verify that the updated inherited expression
	does not contain an internal call to a protected function.  This
	completes the implementation of AI12-0166.
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