Introduce a separate StackValue class for lateinit local vars

Local lateinit var differs in behavior from a simple local var,
and logic that relies of 'instanceof Local' checks assuming that all
instances of Local are simple local vars can produce faulty code
(as in KT-23260, where a local lateinit var was not explicitly put on
stack when passed as an argument to an inline function, thus causing
null propagation).

 #KT-23260 Fixed Target versions 1.2.50

compiler/testData/codegen/box/properties/lateinit/local/kt23260.kt

@@ -0,0 +1,15 @@
+// WITH_RUNTIME
+
+fun box(): String {
+    lateinit var str: String
+    try {
+        println(str)
+        return "Should throw an exception"
+    }
+    catch (e: UninitializedPropertyAccessException) {
+        return "OK"
+    }
+    catch (e: Throwable) {
+        return "Unexpected exception: ${e::class}"
+    }
+}