[Ada] Spurious warning when using 'Type_Key on generic formal

[Pierre-Marie de Rodat via Gcc-patches]

This patch corrects an issue in the compiler whereby spurious
compile-time warnings get raised when the attribute 'Type_Key is applied
to a generic formal type.

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

gcc/ada/

        * sem_attr.adb (Type_Key): Avoid premature use warnings when the
        prefix is a generic actual.
diff --git a/gcc/ada/sem_attr.adb b/gcc/ada/sem_attr.adb
--- a/gcc/ada/sem_attr.adb
+++ b/gcc/ada/sem_attr.adb
@@ -6664,6 +6664,7 @@ package body Sem_Attr is
 
          if not Is_Frozen (Entity (P))
            and then not Is_Generic_Type (Entity (P))
+           and then not Is_Generic_Actual_Type (Entity (P))
          then
             Error_Msg_N ("premature usage of Type_Key?", N);
          end if;