message: Update the name of the swift branch we check out when
  compiling the github lldb swift-4.1-branch sources.
diff: {"name": "swift",
  "vcs": "git",
  "url": "https://github.com/apple/swift.git",
  - "ref": "master"},
  + "ref": "swift-4.1-branch"},
  {"name": "cmark",
  "vcs": "git",
  