The default IndexingIterator for a custom collection is not suitable for
subclasses that specialize the element type the way libSyntax does.
Overriding it with a more specific iterator, even with something like
covariant overrides, would not work because it the iterator is generic
over its base collection, not its element type.
Provide a custom iterator parameterized by the element type instead.
* Create Swift libSyntax API
This patch is an initial implementation of the Swift libSyntax API. It
aims to provide all features of the C++ API but exposed to Swift.
It currently resides in SwiftExperimental and will likely exist in a
molten state for a while.
* Only build SwiftSyntax on macOS
