Improve the ergonomics of working with the Symbol.Availability.Domain constants

[d-ronnqvist]

Bug/issue #, if applicable:

Summary

This improves the ergonomics of creating Symbol.Availability.Domain values for known constants.

Currently, because the constants are string values, creating a domain value for a known domain looks like:

let domain = Symbol.Availability.Domain(rawValue: Symbol.Availability.Domain.macOS)

with these changes, creating the same domain value looks like:

let domain = Symbol.Availability.Domain.macOS

Warning

This is an API breaking change

I tried to avoid making API breaking changes here but I didn't really like any of the alternatives. The least bad workaround I could think of was:

public static let swiftDomain = Domain(rawValue: "Swift")
@available(*, deprecated, renamed: "swiftDomain", message: "Use 'swiftDomain' instead. This deprecated API will be removed after 6.2 is released")
public static let swift = "Swift"

What I don't like about it is that after the 6.2 release we're either going to have API that doesn't follow Swift's API Design Guidelines (because it repeats the type name in the property name) or we're going to have to deprecate it again to rename it back to just "swift".

I have some hopes that this API is nearly unused in practice because the current ergonomics are so bad that I'd expect that someone would have complained about it if anyone was using it.

Dependencies

This doesn't depend on anything but it's a breaking change for other repos.

Testing

Nothing in particular. This isn't a user-facing change.

Checklist

Make sure you check off the following items. If they cannot be completed, provide a reason.

• Added tests
• Ran the ./bin/test script and it succeeded
• Updated documentation if necessary