Change default apple_platform_type to macOS

When using rules for Apple platforms that aren't platform specific, such
as `swift_binary`, the default of iOS doesn't make sense, instead we
should default this to the current platform, which currently can only be
macOS.

src/main/java/com/google/devtools/build/lib/rules/apple/AppleCommandLineOptions.java

@@ -221,7 +221,7 @@ public class AppleCommandLineOptions extends FragmentOptions {
 
   @Option(
     name = "apple_platform_type",
-    defaultValue = "IOS",
+    defaultValue = "MACOS",
     converter = PlatformTypeConverter.class,
     documentationCategory = OptionDocumentationCategory.UNDOCUMENTED,
     effectTags = {OptionEffectTag.BAZEL_INTERNAL_CONFIGURATION},