KT-4893 Code completion should not show multiple functions with the same signature

#KT-4893 Fixed

idea/idea-completion/testData/basic/common/shadowing/PreferMemberExtension.kt

@@ -0,0 +1,19 @@
+interface I
+
+class C : I {
+    inner class Inner {
+        fun foo() {
+            xx<caret>
+        }
+
+        val Any.xxx: Int get() = 1
+    }
+
+    val I.xxx: Int get() = 1
+
+}
+
+val C.xxx: Int get() = 1
+
+// EXIST: { lookupString: "xxx", itemText: "xxx", tailText: " for Any in C.Inner", typeText: "Int" }
+// NOTHING_ELSE