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/frontend/src/org/jetbrains/kotlin/resolve/checkers/ExplicitApiDeclarationChecker.kt

@@ -1,5 +1,5 @@
 /*
- * Copyright 2010-2019 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Copyright 2010-2020 JetBrains s.r.o. and Kotlin Programming Language contributors.
  * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
  */
 
@@ -21,6 +21,7 @@ class ExplicitApiDeclarationChecker : DeclarationChecker {
         if (state == ExplicitApiMode.DISABLED) return
 
         if (descriptor !is DeclarationDescriptorWithVisibility) return
+        if (descriptor is ClassDescriptor && descriptor.kind == ClassKind.ENUM_ENTRY) return // Enum entries does not have visibilities
         if (!descriptor.isEffectivelyPublicApi) return
 
         checkVisibilityModifier(state, declaration, descriptor, context)

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 }