Issue Description:

Similar to source.lang.swift.expr.array and source.lang.swift.expr.dictionary, SourceKit should provide information about tuples inside a structure. I propose a new expression kind source.lang.swift.expr.tuple.

This would provide key.elements as the other mentioned kinds. Those elements should also provide information about elements names.

This new information would make possible for tools like SwiftLint to evaluate this information.

For SwiftLint in special, this would be useful to improve the Large Tuple Rule, as it currently only evaluates tuples in variable or function declarations. It can't catch cases like using tuples as arguments in generic functions. While it'd probably be possible to implement that in the current state, having SourceKit to provide this information would make it much more reliable.