[Clang importer] Generalize open-enums test for all platforms.

(cherry picked from commit 8914ec6)

Author: DougGregor

test/ClangImporter/availability_open_enums_macos.swift renamed to test/ClangImporter/availability_open_enums.swift

@@ -1,6 +1,5 @@
-// RUN: %target-swift-frontend(mock-sdk: %clang-importer-sdk) -typecheck -verify -I %S/Inputs/custom-modules -swift-version 4 -target x86_64-apple-macosx10.13 %s 
+// RUN: %target-swift-frontend(mock-sdk: %clang-importer-sdk) -typecheck -verify -I %S/Inputs/custom-modules -swift-version 4 %s
 
-// REQUIRES: OS=macosx
 // REQUIRES: objc_interop
 
 import Foundation

test/ClangImporter/availability_open_enums_macos_swift3.swift renamed to test/ClangImporter/availability_open_enums_swift3.swift

@@ -1,6 +1,5 @@
-// RUN: %target-swift-frontend(mock-sdk: %clang-importer-sdk) -typecheck -verify -I %S/Inputs/custom-modules -swift-version 3 -target x86_64-apple-macosx10.13 %s 
+// RUN: %target-swift-frontend(mock-sdk: %clang-importer-sdk) -typecheck -verify -I %S/Inputs/custom-modules -swift-version 3 %s
 
-// REQUIRES: OS=macosx
 // REQUIRES: objc_interop
 
 import Foundation