Revert "Temporarily remove isInitialized and tests, but keep the implementation"

This reverts commit 234148518e.

compiler/testData/codegen/box/properties/lateinit/isInitializedAndDeinitialize/topLevelProperty.kt

@@ -0,0 +1,12 @@
+// TARGET_BACKEND: JVM
+// LANGUAGE_VERSION: 1.2
+// WITH_RUNTIME
+
+lateinit var bar: String
+
+fun box(): String {
+    if (::bar.isInitialized) return "Fail 1"
+    bar = "OK"
+    if (!::bar.isInitialized) return "Fail 2"
+    return bar
+}