Fix incorrect UNINITIALIZED_ENUM_COMPANION diagnostic

#KT-20959 Fixed

compiler/testData/diagnostics/tests/enum/wrongUnitializedEnumCompanion.kt

@@ -0,0 +1,9 @@
+// See KT-20959
+
+enum class Foo {;
+    companion object  {
+        val x = <!DEBUG_INFO_LEAKING_THIS!>foo<!>() // there should be no UNINITIALIZED_ENUM_COMPANION
+
+        private fun foo() = "OK"
+    }
+}