This patch also performs minor refactoring to align syntax parsing
context with the right scope. We start to support the generic clauses
because they are necessary pieces to construct struct or
function syntax node.
* 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
