This patch provides an API and Serialization for clang-style diagnostics
from within Swift. Libraries can use this API to pop their own custom
diagnostics that can be serialized to JSON.
* 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
