Resolve extension calls on class objects

#KT-3470 Fixed

compiler/testData/diagnostics/tests/extensions/classObject.kt

@@ -0,0 +1,19 @@
+trait Tr
+
+class A { class object }
+class B { class object : Tr }
+
+fun Any.f1() {}
+fun Any?.f2() {}
+fun Tr.f3() {}
+fun Tr?.f4() {}
+fun A.f5() {}
+
+fun test() {
+    A.f1()
+    A.f2()
+    B.f3()
+    B.f4()
+    A.<!UNRESOLVED_REFERENCE_WRONG_RECEIVER!>f5<!>()
+    B.<!UNRESOLVED_REFERENCE_WRONG_RECEIVER!>f5<!>()
+}

compiler/testData/diagnostics/tests/extensions/kt3470.kt

@@ -0,0 +1,7 @@
+class A {
+    class object {
+        fun foo() = toString()
+    }
+}
+
+val a = A.toString()

compiler/testData/diagnostics/tests/extensions/noClassObjectsInJava.kt

@@ -0,0 +1,11 @@
+// FILE: A.java
+public class A {
+    public static void foo() {}
+    public static class Nested {}
+}
+
+// FILE: B.kt
+fun Any?.bar() = 42
+
+fun f1() = A.<!UNRESOLVED_REFERENCE!>bar<!>()
+fun f2() = A.Nested.<!UNRESOLVED_REFERENCE!>bar<!>()