Fix retrieving documentation for overridden methods.

(The test doesn't actually reproduce the problem because it can't be done in a single-file setup, but it verifies that the basic functionality continues to work.)

idea/testData/kdoc/finder/Overridden.kt

@@ -0,0 +1,10 @@
+open class Foo() {
+    /**
+     * Doc for method xyzzy
+     */
+    open fun xyzzy(): Int = 0
+}
+
+open class Bar(): Foo() {
+    override fun xyzzy(): Int = 1
+}