Initial implementation of KT-5955 Completion for override methods

#KT-5955 Fixed

idea/idea-completion/testData/handlers/basic/override/ImplementFunction.kt

@@ -0,0 +1,9 @@
+interface I {
+    fun foo(p: Int)
+}
+
+class A : I {
+    o<caret>
+}
+
+// ELEMENT_TEXT: "override fun foo(p: Int) {...}"