[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  <schonberg@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.
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