[Ada] Crash on pragma Compile_Time_Warning with declared string constant

[Ada] Crash on pragma Compile_Time_Warning with declared string constant

This patch fixes a compiler abort on a pragma Compile_Time_Warning when its
second argument is a reference to a constsant string (rather than a string
literal or an expression that evaluates to a string literal).

Compiling msain.adb must yield:

   main.adb:5:33: warning: Good
   main.adb:6:33: warning: VALLUE
   main.adb:7:33: warning: Test

----
procedure Main is
   Value : constant String := "Test";
   Switch : constant Boolean := True;
begin
   pragma Compile_Time_Warning (Switch, "Good");
   pragma Compile_Time_Warning (Switch, "VAL" & "LUE");
   pragma Compile_Time_Warning (Switch, value);
   null;
end Main;

2018-05-22  Ed Schonberg  <schonberg@adacore.com>

gcc/ada/

	* sem_prag.adb (Process_Compile_Time_Warning_Or_Error): Handle properly
	a second argument that is a constant of a given string value.

From-SVN: r260514