Merge pull request swiftlang#228 from apple/update-swiftpm-branch

Use swift-4.2-branch of swift-package-manager

Author: Zachary 'Clack' Cole

common.py

@@ -52,7 +52,7 @@
         'cmark': 'master',
         'ninja': 'release',
         'llbuild': 'master',
-        'swiftpm': 'master',
+        'swiftpm': 'swift-4.2-branch',
         'swift-corelibs-libdispatch': 'swift-4.2-branch',
         'swift-corelibs-foundation': 'swift-4.2-branch',
         'swift-corelibs-xctest': 'swift-4.2-branch',