KT-3750 When without else

#KT-3750 Fixed

idea/testData/checker/WhenInEnumInExtensionProperty.kt

@@ -0,0 +1,15 @@
+// KT-3750 When without else
+
+enum class A {
+    e1
+    e2
+}
+class B(val a: A)
+val B.foo: Int
+    get() {
+        // Check absence [NO_ELSE_IN_WHEN] error
+        return when (a) {
+            A.e1 -> 1
+            A.e2 -> 3
+        }
+    }