KT-5079 Support smart completion for when values

#KT-5079 Fixed

idea/testData/completion/smart/WhenEntryValue4.kt

@@ -0,0 +1,17 @@
+enum class E {
+    A
+    B
+    C
+}
+
+fun foo(e: E) {
+    when(e) {
+        <caret>
+        else -> x()
+    }
+}
+
+// EXIST: E.A
+// EXIST: E.B
+// EXIST: E.C
+// ABSENT: else