Support KClass.objectInstance for objects and companion objects

compiler/testData/codegen/boxWithStdlib/reflection/classes/objectInstance.kt

@@ -0,0 +1,29 @@
+import kotlin.test.assertEquals
+
+object Obj {
+    fun foo() = 1
+}
+
+class A {
+    companion object {
+        fun foo() = 2
+    }
+}
+
+class B {
+    companion object Factory {
+        fun foo() = 3
+    }
+}
+
+class C
+
+fun box(): String {
+    assertEquals(1, Obj::class.objectInstance!!.foo())
+    assertEquals(2, A.Companion::class.objectInstance!!.foo())
+    assertEquals(3, B.Factory::class.objectInstance!!.foo())
+
+    assertEquals(null, C::class.objectInstance)
+
+    return "OK"
+}