Some structures of syntax nodes can have children choices, e.g. a
dictionary expression can either contain a single ':' token or a list of
key-value pairs.
This patch gives the existing code generation infrastructure a way to
specify such node choices. Node choices are specified under a child
declaration with two constraints: a choice cannot be declared as
optional, and a choice cannot have further recursive choices.
Since we don't have too many node structures with choices, part of the
SyntaxFactory code for these nodes is manually typed.
This patch also teaches AccessorBlock to use node choices.
* 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
* 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.
