KT-5079 Support smart completion for when values

#KT-5079 Fixed

idea/testData/completion/smart/WhenEntryValue6.kt

@@ -0,0 +1,18 @@
+enum class E {
+  A
+  B
+  C
+}
+
+fun foo(e: E?) {
+    when(e) {
+        E.A -> x()
+        <caret>
+    }
+}
+
+// ABSENT: E.A
+// EXIST: E.B
+// EXIST: E.C
+// EXIST: null
+// EXIST: {"lookupString":"else","tailText":" ->","itemText":"else"}