Instead, reference count the SyntaxData's parent. This has a couple of
advantages:
1. We eliminate a const_cast that was potentially unsafe
2. It more closely resembles the architecture on the Swift side
3. It has the potential to be optimised further if the parent can be
accessed in an unsafe, non-reference-counted way
* 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
