Proposed changes

Allows a subset of argument_tags for the SourcesComputer to be fetched from the const_global_cache. This isn't used for anything right now, but will be used in an upcoming system.

Upgrade instructions

All Sources and LinearizedSources classes must now define the alias const_global_cache_tags.

Code review checklist

• The code is documented and the documentation renders correctly. Run
  make doc to generate the documentation locally into BUILD_DIR/docs/html.
  Then open index.html.
• The code follows the stylistic and code quality guidelines listed in the
  code review guide.
• The PR lists upgrade instructions and is labeled bugfix or
  new feature if appropriate.

Further comments