update-checkout: use semantic revision

[compnerd]

Use the semantic name (v2.1.0-rc1) instead of an explicit ref to make it
easier to identify the revision in use.

Replace this paragraph with a description of your changes and rationale. Provide links to external references/discussions if appropriate.

Resolves SR-NNNN.

[compnerd]

@swift-ci please test tensorflow

[marcrasi]

Sometimes we have to clean test when we change the update-checkout-config.json. I'll try triggering that.

[marcrasi]

@swift-ci please clean test tensorflow

[dan-zheng]

CI fails with this error:

Updating '/home/swiftci/jenkins/workspace/swift-PR-TensorFlow-Linux/swift-tools-support-core'
Error on repo "/home/swiftci/jenkins/workspace/swift-PR-TensorFlow-Linux/swift-tools-support-core": Traceback (most recent call last):
  File "/home/swiftci/jenkins/workspace/swift-PR-TensorFlow-Linux/swift/utils/update_checkout/update_checkout/update_checkout.py", line 91, in update_single_repository
    config, repo_name, scheme_name, scheme_map, cross_repos_pr)
  File "/home/swiftci/jenkins/workspace/swift-PR-TensorFlow-Linux/swift/utils/update_checkout/update_checkout/update_checkout.py", line 58, in get_branch_for_repo
    scheme_branch = scheme_map[repo_name]
KeyError: u'swift-tools-support-core'

This error occurs because CI was last triggered for the tensorflow-merge -> tensorflow PR #28847, which involved changes to update-checkout-config.json.

Sometimes, CI machines are left in an incompatible state after testing tensorflow-merge -> tensorflow PRs, so a clean test (@swift-ci Please clean test tensorflow) is necessary.

This error should naturally go away when #28847 is merged soon today. I'd recommend waiting for that PR to be merged before this one, to unblock the master -> tensorflow merge and to avoid merge conflicts.

[compnerd]

@swift-ci Please clean test tensorflow

[compnerd]

@swift-ci please clean test tensorflow

[dan-zheng]

@swift-ci Please clean test tensorflow

[dan-zheng]

CI fails:

You have bazel 0.26.1 installed.
Please upgrade your bazel installation to version 0.27.1 or higher to build TensorFlow!

I'll upgrade Bazel on all CI machines to 0.29.1 (the max supported version) now.

[dan-zheng]

@swift-ci Please test tensorflow