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" }

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

@@ -0,0 +1,22 @@
+interface I {
+    fun foo()
+    val someVal: Int
+    var someVar: Int
+}
+
+class Base1 {
+    protected open fun bar(){}
+    open val fromBase: String = ""
+}
+
+open class Base2 : Base1() {
+}
+
+class A(overr<caret>) : Base2(), I
+
+// EXIST: { lookupString: "override", itemText: "override" }
+// EXIST: { itemText: "override val someVal: Int", tailText: null, typeText: "I" }
+// EXIST: { itemText: "override var someVar: Int", tailText: null, typeText: "I" }
+// EXIST: { itemText: "override val fromBase: String", tailText: null, typeText: "Base1" }
+// EXIST_JAVA_ONLY: { lookupString: "override", itemText: "override: Override" }
+// NOTHING_ELSE

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

@@ -0,0 +1,24 @@
+interface I {
+    fun foo()
+    val someVal: Int
+    var someVar: Int
+}
+
+class Base1 {
+    protected open fun bar(){}
+}
+
+open class Base2 : Base1() {
+}
+
+class A : Base2(), I {
+    o<caret>
+}
+
+// EXIST: { lookupString: "override", itemText: "override" }
+// EXIST: { itemText: "override fun bar() {...}", lookupString: "override", tailText: null, typeText: "Base1" }
+// EXIST: { itemText: "override fun equals(other: Any?): Boolean {...}", lookupString: "override", tailText: null, typeText: "Any" }
+// EXIST: { itemText: "override fun foo() {...}", lookupString: "override", tailText: null, typeText: "I" }
+// EXIST: { itemText: "override fun hashCode(): Int {...}", lookupString: "override", tailText: null, typeText: "Any" }
+// EXIST: { itemText: "override val someVal: Int", lookupString: "override", tailText: null, typeText: "I" }
+// EXIST: { itemText: "override var someVar: Int", lookupString: "override", tailText: null, typeText: "I" }