[Ada] Crash processing Valid_Scalars whose evaluation is always true

The compiler blows up generating code associated with occurrences of attribute
Valid_Scalars whose evaluation is always true. After this patch the following
test compiles fine.

2018-05-23  Javier Miranda  <miranda@adacore.com>

gcc/ada/

	* sem_attr.adb (Valid_Scalars): Do not invoke Error_Attr_P to report
	the warning on occurrences of this attribute whose evaluation is always
	true (since that subprogram aborts processing the attribute). In
	addition, replace the node by its boolean result 'True' (required
	because the backend has no knowledge of this attribute).

gcc/testsuite/

	* gnat.dg/valid_scalars1.adb: New testcase.

From-SVN: r260591

gcc/ada/ChangeLog

+2018-05-23  Javier Miranda  <miranda@adacore.com>
+
+	* sem_attr.adb (Valid_Scalars): Do not invoke Error_Attr_P to report
+	the warning on occurrences of this attribute whose evaluation is always
+	true (since that subprogram aborts processing the attribute). In
+	addition, replace the node by its boolean result 'True' (required
+	because the backend has no knowledge of this attribute).
+
 2018-05-23  Bob Duff  <duff@adacore.com>
 
 	* libgnat/a-convec.adb: (Insert, Insert_Space): Suppress warnings. The

gcc/ada/sem_attr.adb

@@ -6929,8 +6929,10 @@ package body Sem_Attr is
 
             else
                if not Scalar_Part_Present (P_Type) then
-                  Error_Attr_P
-                    ("??attribute % always True, no scalars to check");
+                  Error_Msg_Name_1 := Aname;
+                  Error_Msg_F
+                    ("??attribute % always True, no scalars to check", P);
+                  Set_Boolean_Result (N, True);
                end if;
 
                --  Attribute 'Valid_Scalars is illegal on unchecked union types

gcc/testsuite/ChangeLog

+2018-05-23  Javier Miranda  <miranda@adacore.com>
+
+	* gnat.dg/valid_scalars1.adb: New testcase.
+
 2018-05-23  Ed Schonberg  <schonberg@adacore.com>
 
 	* gnat.dg/iter1.adb, gnat.dg/iter1.ads: New testcase.

gcc/testsuite/gnat.dg/valid_scalars1.adb

+--  { dg-do compile }
+--  { dg-options "-gnata -gnatws" }
+
+procedure Valid_Scalars1 is
+   type Ptr is access Integer;
+   V1 : Ptr;
+
+   Check : Boolean := V1'Valid_Scalars;
+begin
+   pragma Assert (Check);
+end;