[Ada] Reject properly an aspect Predicate on a formal type

[Pierre-Marie de Rodat]

This patch adds a check to reject an aspect Predicate on a formal type
declaration.

Compiling gen.adb must yield:

   gen.ads:2:55: predicate cannot apply to formal type

---
generic
   type T is array (Integer range <>) of Integer
       with Predicate => T'First /= 1;
function Gen return Integer;
---
function Gen return Integer is
   X : T(1 .. 2);
begin
   return X'First;
end Gen;

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

2017-12-05  Ed Schonberg  <schonb...@adacore.com>

        * sem_ch13.adb (Analyze_Aspect_Specifications, case Predicate): A
        predicate cannot apply to a formal type.


Index: sem_ch13.adb
===================================================================
--- sem_ch13.adb        (revision 255408)
+++ sem_ch13.adb        (working copy)
@@ -2389,6 +2389,10 @@
                   elsif Is_Incomplete_Type (E) then
                      Error_Msg_N
                        ("predicate cannot apply to incomplete view", Aspect);
+
+                  elsif Is_Generic_Type (E) then
+                     Error_Msg_N
+                       ("predicate cannot apply to formal type", Aspect);
                      goto Continue;
                   end if;