KT-5079 Support smart completion for when values

#KT-5079 Fixed

idea/testData/completion/smart/WhenEntryValue2.kt

@@ -0,0 +1,16 @@
+enum class E {
+    A
+    B
+    C
+}
+
+fun foo(e: E) {
+    when(e) {
+        E.A, <caret>
+    }
+}
+
+// ABSENT: E.A
+// EXIST: E.B
+// EXIST: E.C
+// ABSENT:else