extractschema: Include common node facts in the result.
extractschema: Include common node facts in the result.

Summary:
Add a new section to the result message that includes the common node facts
shared by various nodes in the schema, regardless of kind.

Reviewers: danielmoy-google, #core_team

Reviewed By: danielmoy-google, #core_team

Subscribers: #core_team

Differential Revision: https://phabricator-dot-kythe-repo.appspot.com/D2092

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