Update the Swift parser to Swift 5.1.

[michelle-joudrey]

We're probably still using the Swift 3.0 parser since it's never been updated.
https://github.com/rjoudrey/swift

We are currently distributing a whole lot of unnecessary cmark/llvm/swift static libraries in the lib folder. We should take this opportunity to build a single standalone static library for the extension project. This should reduce the lib folder size a bit since we won't be including a bunch of unused static library code.