Feature Request: Replay Swift parseable output

[brentleyjones]

Once rules_swift has the ability to generate serialized diagnostics, when building with Bazel, we should:

• Supply the needed feature in the _xcodeproj_transition (if it uses a feature)
• Add the dia and parseable output files to a targets output group in output_files.bzl for both build and index
• Copy over these files as we do other outputs, to the location expected by Xcode
• cat the parsable output in stubs/swiftc.py

[brentleyjones]

rules_swift will need an updated version of this: bazelbuild/rules_swift#304