* [SwiftSyntax] Fix SyntaxChildren iteration
SyntaxChildren.Iterator stops the first time it sees a nil child;
since it traverses all indexes without considering presence, this
causes it to terminate iteration early if there is a missing child
in the list.
* [SwiftSyntax] Add tests for SyntaxChildren
* 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