Add basic Swift support (requires Xcode 6)

By default expressions are evaluated in current frame's language.

When stopped inside a Swift function, current language is Swift, and it's invalid to use Objective-C syntax: many Chisel functions break as a result.

Explicitly specify Objective-C++ as expression language when evaluating commands.

However one problem will still persist: it's still impossible to use Swift objects by name in Chisel functions. One needs to first convert object names to id pointers using evaluateExpressionValueInFrameLanguage(), but the lldb code to enable that was committed very recently and is still unavailable in Xcode 6.1.1.