This adds a couple of changes to support building SwiftSyntax without an ObjC
runtime as well as swift-corelibs-foundation instead of the ObjC Foundation
implemetation. This is needed to support building SwiftSyntax on Linux.
* 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