KT-1985 Add test case and fix minor bug

Reference compacting must be done before adding body to class.

idea/testData/codeInsight/overrideImplement/emptyClassBodyFunctionMethod.kt.after

@@ -0,0 +1,10 @@
+// From KT-1254
+trait T {
+    fun Foo() : (String) -> Unit
+}
+
+class C : T {
+    override fun Foo(): (String) -> Unit {
+        throw UnsupportedOperationException()
+    }
+}