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

#KT-4893 Fixed

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

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