* Generate libSyntax API
This patch removes the hand-rolled libSyntax API and replaces it with an
API that's entirely automatically generated. This means the API is
guaranteed to be internally stylistically and functionally consistent.
This will make it easier to incrementally implement syntax nodes,
while allowing us to embed nodes that we do know about inside ones
that we don't.
https://bugs.swift.org/browse/SR-4062