It's relatively complex to determine in advance if a Xcode, SDK,
and Deployment Target configuration is capable of running Swift.
For example the following combinations do not work:
* deployment target < OS X 10.9
* Xcode 6.2 and macosx10.9 SDK
* Xcode 7 Beta 6 and macosx10.10 SDK
Until we found out how to query Xcode for Swift support in a reliable
way, the RunCMake.Swift test cases will be restricted to negative ones.
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`.
Gregor Jasny
Brad King