AST: Introduce Builtin.AnyObject

Soon, Swift.AnyObject will become a protocol composition type
with no protocols and a class constraint. Since there's no
way to write this in the language, cons up an AnyObject type
in the Builtin module, so that the standard library can define
'typealias AnyObject = Builtin.AnyObject'.

lib/AST/Builtins.cpp

@@ -109,6 +109,12 @@ Type swift::getBuiltinType(ASTContext &Context, StringRef Name) {
   if (Name == "FPPPC128")
     return Context.ThePPC128Type;
 
+  // AnyObject is the empty class-constrained existential.
+  if (Name == "AnyObject")
+    return CanType(
+      ProtocolCompositionType::get(Context, {},
+                                   /*hasExplicitAnyObject=*/true));
+
   return Type();
 }