swift234

Add overloads for range types to IndexSet

The Swift 3 refactoring of the range type has led to its split into 4
different types. The IndexSet API should accept any of these as long as
they contain the element type (Int, which is inherently Countable). This
allows callers to use both the ... and ..< syntax, for example.

This commit also adds additional unit tests for some of the IndexSet
API, and turns a few methods with optional/default args into properties
or a method family, since otherwise callers would end up with an
ambigious method call as the range argument would have been defaulted to
nil.

<rdar://problem/26532614> Add overloads for range types to IndexSet