Generate equals/hashCode(): Enable for classes without properties

Also:
- forbid for enum classes
- use javaClass.hashCode() instead of 0 if class has no properties

 #KT-18418 Fixed

idea/testData/codeInsight/generate/equalsWithHashCode/noVars.kt.after

@@ -0,0 +1,15 @@
+class A {
+    fun foo() {
+
+    }
+
+    <caret>override fun equals(other: Any?): Boolean {
+        if (this === other) return true
+        if (other?.javaClass != javaClass) return false
+        return true
+    }
+
+    override fun hashCode(): Int {
+        return javaClass.hashCode()
+    }
+}