Decide how to support `cxxLanguageStandard` in a Swift manifest.

[cgrindel]

The problem is that we do not want to add it to the cxxopts for each target. It would override the values in the .bazelrc. Also, I think that you do not want modules linked with different standards.

• Should we output cxxLanguageStandard values as we come across them?
• Should we scan all of the manifests and pick the highest one?
• How should we tell the client?
  • We can print the value(s) from Starlark, but they could be easily missed in the output.
  • We can analyze the manifests during update-repos and print a suggested .bazelrc entry.

Example:

• firebase/abseil-cpp-SwiftPM