Create a new CMAKE_Swift_LANGUAGE_VERSION variable to specify the
SWIFT_VERSION attribute in a generated Xcode project. Ideally this
would be a `<LANG>_STANDARD` property but since Swift support is
very minimal we should reserve that property for more complete
treatment later.
Issue: #16326
Allow the `Swift` language to be enabled with the Xcode generator for
Xcode >= 6.1. Reject it on other generators and with older Xcode
versions. Since Apple is the only vendor implementing the language
right now, the compiler id can be just `Apple`.