When using SwiftSyntax as a standalone tool, we invoke Swiftc
internally to get serialized syntax trees. This is not ideal for
several reasons: (1) we have to hard-code the relative path of swiftc
to invoke it; (2) we have to rely on standard input/output to pass the
tree across the process boundaries; and (3) we have to maintain two
different ways to get syntax tree (swiftc and sourcekitd).
This patch attempts to teach SwiftSyntax to use SourceKitd to get the
tree just like other clients. We first add a SourceKitd client library
written in Swift; and next teach SwiftSyntax to adopt this SourceKitd
client-side library. For platforms other than MacOS, we still use Swiftc
to get syntax trees. This client library also allows us to add
SourceKitd tests in Swift.
This patch also re-enables several flaky tests.
