KT-2714 'enum' annotation should only be allowed on classes

#KT-2714 Fixed

compiler/testData/diagnostics/tests/enum/enumModifier.kt

@@ -0,0 +1,15 @@
+enum class B {}
+
+class A {
+   <!ILLEGAL_ENUM_ANNOTATION!>enum<!> class object {}
+}
+
+<!ILLEGAL_ENUM_ANNOTATION!>enum<!> object O {}
+
+<!ILLEGAL_ENUM_ANNOTATION!>enum<!> trait T {}
+
+<!ILLEGAL_ENUM_ANNOTATION!>enum<!> fun f() = 0
+
+<!ILLEGAL_ENUM_ANNOTATION!>enum<!> val x = 0
+
+<!ILLEGAL_ENUM_ANNOTATION!>enum<!> var y = 0