store nullability information in TypeInfo

idea/testData/codegen/typeInfo/nullability.jet

@@ -0,0 +1,13 @@
+class Box<T>(t: T) {
+    var value = t
+}
+
+fun box(): String {
+  val nullableBox = Box<String?>("")
+  val notnullBox = Box<String>("")
+  if (nullableBox is Box<String>) return "fail 1"
+  if (notnullBox is Box<String?>) return "fail 2"
+  if (nullableBox !is Box<String?>) return "fail 3"
+  if (notnullBox !is Box<String>) return "fail 4"
+  return "OK"
+}