KT-5079 Support smart completion for when values

#KT-5079 Fixed

idea/testData/completion/handlers/smart/WhenElse.kt

@@ -0,0 +1,7 @@
+fun foo(s: String) {
+    when(s) {
+        <caret>
+    }
+}
+
+// ELEMENT: else

idea/testData/completion/handlers/smart/WhenElse.kt.after

@@ -0,0 +1,7 @@
+fun foo(s: String) {
+    when(s) {
+        else -> <caret>
+    }
+}
+
+// ELEMENT: else