Do not check visibility of enum entries in explicit api mode

Although they're instances of DeclarationDescriptorWithVisibility,
they can't have visibilities

#KT-37040 Fixed

compiler/testData/diagnostics/testsWithExplicitApi/classes.kt

@@ -39,6 +39,11 @@ public data class FooData(val i: Int, val s: String)
 <!NO_EXPLICIT_VISIBILITY_IN_API_MODE!>data class FooData2<!>(val i: Int, val s: String)
 
 public class WithNested {
-    <!NO_EXPLICIT_VISIBILITY_IN_API_MODE!>class Nested<!> {}
-    <!NO_EXPLICIT_VISIBILITY_IN_API_MODE!>inner class Inner<!> {}
-}
+    <!NO_EXPLICIT_VISIBILITY_IN_API_MODE!>
+    class Nested<!> {}
+    <!NO_EXPLICIT_VISIBILITY_IN_API_MODE!>
+    inner class Inner<!> {}
+}
+
+<!NO_EXPLICIT_VISIBILITY_IN_API_MODE!>enum class Foo<!> { A, B }
+public enum class Bar { A, B }