Fix initialization of object's INSTANCE$ field

Call object's constructor in <clinit> and ignore the result, but in the first
line of <init> save the current "this" to INSTANCE$

 #KT-4516 Fixed
 #KT-4543 Fixed
 #KT-5291 Fixed
 #KT-5523 Fixed
 #KT-5582 Fixed

compiler/testData/codegen/box/objects/privateExtensionFromInitializer_kt4543.kt

@@ -0,0 +1,16 @@
+class A(val result: String)
+
+fun a(body: A.() -> String): String {
+    val r = A("OK")
+    return r.body()
+}
+
+object C {
+    private fun A.f() = result
+
+    val g = a {
+        f()
+    }
+}
+
+fun box() = C.g