Make 'when' on nullable enums exhaustive if 'null' entry is present

#KT-2902 Fixed

compiler/testData/diagnostics/tests/when/ElseOnNullableEnumWithSmartCast.kt

@@ -0,0 +1,11 @@
+enum class E { A B }
+
+fun foo(e: E, something: Any?): Int {
+    if (something != null) return 0
+
+    return when (e) {
+        E.A -> 1
+        E.B -> 2
+        something -> 3
+    }
+}