[Ada] Static predicate check on characters of a string literal

[Pierre-Marie de Rodat]

This patch implements the rule given in RM 4.2 (11): if the component type of
a string literal is a character type with a static predicate, that predicate
must be applied to each character in the string.

Compiling the example below must yield:

   gcc -c -gnata main.adb

  main.adb:4:23: warning: static expression fails static predicate check on "C"
  main.adb:4:23: warning: expression is no longer considered static
  main.adb:4:24: warning: static expression fails static predicate check on "C"
  main.adb:4:24: warning: expression is no longer considered static
  main.adb:4:25: warning: static expression fails static predicate check on "C"
  main.adb:4:25: warning: expression is no longer considered static

Execution must yield:

  raised SYSTEM.ASSERTIONS.ASSERT_FAILURE :
                                        Static_Predicate failed at main.adb:4
----

procedure Main is
   subtype C is Character with Static_Predicate => C in 'A' | 'B' | 'C';
   type S is array (Positive range <>) of C;
   X : constant S := "abc";
begin
   null;
end;

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

2018-05-31  Ed Schonberg  <schonb...@adacore.com>

gcc/ada/

        * sem_res.adb (Resolve_String_Literal): If the type is a string type
        whose component subtype has a static predicate, ensure that the
        predicate is applied to each character by expanding the string into the
        equivalent aggregate. This is also done if the component subtype is
        constrained.

--- gcc/ada/sem_res.adb
+++ gcc/ada/sem_res.adb
@@ -10774,7 +10774,9 @@ package body Sem_Res is
          --  whether the evaluation of the string will raise constraint error.
          --  Otherwise we need to transform the string literal into the
          --  corresponding character aggregate and let the aggregate code do
-         --  the checking.
+         --  the checking. We use the same transformation if the component
+         --  type has a static predicate, which will be applied to each
+         --  character when the aggregate is resolved.
 
          if Is_Standard_Character_Type (R_Typ) then
 
@@ -10811,7 +10813,9 @@ package body Sem_Res is
                      end if;
                   end loop;
 
-                  return;
+                  if not Has_Static_Predicate (C_Typ) then
+                     return;
+                  end if;
                end if;
             end;
          end if;