Initial implementation of KT-5955 Completion for override methods

#KT-5955 Fixed

idea/idea-completion/testData/basic/common/override/Generics.kt

@@ -0,0 +1,12 @@
+interface I<T> {
+    fun foo(t: T): T
+}
+
+class A<T> : List<String>, I<T> {
+    o<caret>
+}
+
+// EXIST: { lookupString: "override", itemText: "override" }
+// EXIST: { itemText: "override fun hashCode(): Int {...}", tailText: null, typeText: "Any" }
+// EXIST: { itemText: "override fun foo(t: T): T {...}", tailText: null, typeText: "I" }
+// EXIST: { itemText: "override fun get(index: Int): String {...}", tailText: null, typeText: "List" }