Add backstop description for SWIFT_VERSION

[dempseyatgithub]

For some reason, Xcode only has an empty string for the help description of the SWIFT_VERSION build setting.

Add backstop name and description for SWIFT_VERSION.

Also, since the backstop will be overwritten by the empty string, remove entries with empty string values before adding them to the setting info dictionary.

Note that this addresses one of the issues mentioned in #28