Updating swift-syntax to release tag

[ematejska]

@dan-zheng noticed that swift-syntax has release tags now.

[ematejska]

@swift-ci Please test TensorFlow

[dan-zheng]

Context: this fixes macOS toolchain builds. Without updating swift-syntax, there's an error:

** Building SwiftSyntax **
/Users/swiftninjas/s4tf/swift/swift-nightly-install/Library/Developer/Toolchains/swift-tensorflow-DEVELOPMENT-2020-05-01-a.xctoolchain/usr/usr/bin/swift build --package-path /Users/swiftninjas/s4tf/swift-syntax --configuration release --build-path /Users/swiftninjas/s4tf/build/buildbot_osx/unified-swiftpm-build-macosx-x86_64 --multiroot-data-file /Users/swiftninjas/s4tf/swift/utils/build_swift/resources/SwiftPM-Unified-Build.xcworkspace --verbose --product SwiftSyntax
Traceback (most recent call last):
  File "/Users/swiftninjas/s4tf/swift-syntax/build-script.py", line 605, in <module>
    main()
  File "/Users/swiftninjas/s4tf/swift-syntax/build-script.py", line 572, in main
    builder.build('SwiftSyntax', module_group_path=GROUP_INFO_PATH)
  File "/Users/swiftninjas/s4tf/swift-syntax/build-script.py", line 245, in build
    check_call(command, env=env, verbose=self.verbose)
  File "/Users/swiftninjas/s4tf/swift-syntax/build-script.py", line 73, in check_call
    return subprocess.check_call(cmd, cwd=cwd, env=env, stderr=subprocess.STDOUT)
  File "/usr/local/Cellar/python@2/2.7.16_1/Frameworks/Python.framework/Versions/2.7/lib/python2.7/subprocess.py", line 185, in check_call
    retcode = call(*popenargs, **kwargs)
  File "/usr/local/Cellar/python@2/2.7.16_1/Frameworks/Python.framework/Versions/2.7/lib/python2.7/subprocess.py", line 172, in call
    return Popen(*popenargs, **kwargs).wait()
  File "/usr/local/Cellar/python@2/2.7.16_1/Frameworks/Python.framework/Versions/2.7/lib/python2.7/subprocess.py", line 394, in __init__
    errread, errwrite)
  File "/usr/local/Cellar/python@2/2.7.16_1/Frameworks/Python.framework/Versions/2.7/lib/python2.7/subprocess.py", line 1047, in _execute_child
    raise child_exception
OSError: [Errno 2] No such file or directory

[dan-zheng]

CI doesn't test swift-syntax, so this should be safe to merge.

Locally verified that this fixes macOS toolchain build issues regarding swift-syntax. There still seem to be other issues though.