[Ada] Don't accept illegal (e.g., Integer'(null)) generic actuals

[Ada] Don't accept illegal (e.g., Integer'(null)) generic actuals

Sem_Util.Wrong_Type usually emits an error message, but in some cases it
does not. The code which prevents emitting an error message was going
too far in some cases, causing illegal constructs to be accepted. For
example, a qualified expression such as Integer'(null) might be passed
in as an actual parameter in an instantiation of a generic and generate
no error message.

Running this command:

  gcc -c inst.ads

On the following sources:

package Inst is
   type Ptr is new Integer;

   generic
      type TElement is private;
      NonDefini : TElement;
   package ArbMgr is
   end ArbMgr;

   package Pack is new ArbMgr (Ptr, Ptr'(null));

   procedure Dummy;
end Inst;

Should produce the following output:

  inst.ads:10:42: expected type "Ptr" defined at line 2
  inst.ads:10:42: found an access type
  compilation abandoned due to previous error

2019-09-17  Steve Baird  <baird@adacore.com>

gcc/ada/

	* sem_util.adb (Wrong_Type): In deciding to suppress a message,
	it is not enough for In_Instance to be True; in addition,
	In_Generic_Actual (Expr) must be False.
	* sem_type.adb (In_Generic_Actual): Fix bug where traversal of
	parents skips every other node.

From-SVN: r275786