Parcelize: Add an intrinsic to access Parcelable CREATOR fields

Fixes KT-19853.

The CREATOR field is a static field on a Parcelable class which is not
visible from Kotlin since the necessary metadata would have to be in a
Companion object which may not exist.

This commit adds a [parcelableCreator] function to kotlinx.parcelize,
which is optimized to a direct field access whenever possible.

plugins/parcelize/parcelize-compiler/testData/codegen/parcelableCreator.kt

@@ -0,0 +1,28 @@
+// CURIOUS_ABOUT test1, test2, test3
+// WITH_RUNTIME
+
+import kotlinx.parcelize.*
+import android.os.Parcelable
+
+@Parcelize
+class A(val value: Int) : Parcelable
+
+@Parcelize
+class B : Parcelable
+
+@Parcelize
+object C : Parcelable
+
+fun test1() {
+    parcelableCreator<A>()
+    parcelableCreator<B>()
+    parcelableCreator<C>()
+}
+
+inline fun <reified T : Parcelable> test2() {
+    parcelableCreator<T>()
+}
+
+fun test3() {
+    test2<A>()
+}