Model the macOS/OSX alias through the guard attributes

[buyaa-n]

In 6.0 we have updated the PCA analyzer to use the guard attributes on the OperatingSystem.IsXYZ methods in order to communicate the platform compat relationship between platforms, this approach can be used for macOS/OSX instead of hard coding the relation. This could include the following steps:

1. Add SupportedOSPlatformGuard("OSX") to the OperatingSystem.IsMacOS() to express the relation OSX is MacOS
2. Figure how to add the MacOS is OSX relation
3. Figure out how to handle the backward compatibility and remove the hardcoded macOS/OSX relation in the analyzer

Originally posted by @jeffhandley in #53084 (comment)