[Ada] Add missing guard before call to Interface_Present_In_Ancestor

[Pierre-Marie de Rodat via Gcc-patches]

Calling the function on an unspecified type may trigger the failure of
the precondition of the Interfaces accessor.

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

gcc/ada/

        * sem_type.adb (Specific_Type): Check that the type is tagged
        before calling Interface_Present_In_Ancestor on it.
diff --git a/gcc/ada/sem_type.adb b/gcc/ada/sem_type.adb
--- a/gcc/ada/sem_type.adb
+++ b/gcc/ada/sem_type.adb
@@ -3424,7 +3424,8 @@ package body Sem_Type is
       --  Ada 2005 (AI-251): T1 is a concrete type that implements the
       --  class-wide interface T2
 
-      elsif Is_Class_Wide_Type (T2)
+      elsif Is_Tagged_Type (T1)
+        and then Is_Class_Wide_Type (T2)
         and then Is_Interface (Etype (T2))
         and then Interface_Present_In_Ancestor (Typ   => T1,
                                                 Iface => Etype (T2))