[Ada] Crash processing Valid_Scalars whose evaluation is always true

[Pierre-Marie de Rodat]

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.

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

2018-05-23  Javier Miranda  <mira...@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.
--- gcc/ada/sem_attr.adb
+++ 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

--- /dev/null
new file mode 100644
+++ gcc/testsuite/gnat.dg/valid_scalars1.adb
@@ -0,0 +1,11 @@
+--  { 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;