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-runtime/src/kotlinx/parcelize/parcelableCreator.kt

@@ -0,0 +1,18 @@
+/*
+ * Copyright 2010-2021 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+
+@file:Suppress("unused")
+package kotlinx.parcelize
+
+import android.os.Parcelable
+
+/**
+ * Read the CREATOR field of the given [Parcelable] class. Calls to this function with
+ * a concrete class will be optimized to a direct field access.
+ */
+@Suppress("UNCHECKED_CAST")
+inline fun <reified T : Parcelable> parcelableCreator(): Parcelable.Creator<T> =
+    T::class.java.getDeclaredField("CREATOR").get(null) as? Parcelable.Creator<T>
+        ?: throw IllegalArgumentException("Could not access CREATOR field in class ${T::class.simpleName}")