Reduce verbosity of logging in the C++ extractor.
Reduce verbosity of logging in the C++ extractor.

Summary:
It's not necessary to explicitly log each file touched during extraction.  It's
useful for debugging, but make it a VLOG rather than LOG(INFO).

Reviewers: #core_team, shahms

Reviewed By: #core_team, shahms

Subscribers: #core_team

Differential Revision: https://phabricator-dot-kythe-repo.appspot.com/D1431

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