Reflection: add KType.classifier

#KT-8998 In Progress

core/builtins/src/kotlin/reflect/KType.kt

@@ -21,6 +21,14 @@ package kotlin.reflect
  * or a type parameter of some declaration, plus nullability.
  */
 public interface KType {
+    /**
+     * The declaration of the classifier used in this type.
+     * For example, in the type `List<String>` the classifier would be the [KClass] instance for [List].
+     *
+     * Returns `null` if this type is not denotable in Kotlin, for example if it is an intersection type.
+     */
+    public val classifier: KClassifier?
+
     /**
      * `true` if this type was marked nullable in the source code.
      *