message: Update branch names for llvm/clang to correct swift-5.0-branch.
diff: {"name": "llvm",
  "vcs": "git",
  "url": "https://github.com/apple/swift-llvm.git",
  - "ref": "swift-4.2-branch"},
  + "ref": "swift-5.0-branch"},
  {"name": "clang",
  "vcs": "git",
  "url": "https://github.com/apple/swift-clang.git",
  - "ref": "swift-4.2-branch"},
  + "ref": "swift-5.0-branch"},
  {"name": "swift",
  "vcs": "git",
  