Declare lambda return to satisfy older Clang versions.
Declare lambda return to satisfy older Clang versions.

Summary:
Clang 3.5 is not happy with the deduced return type on this particular lambda
expression; but declaring what we want explicitly mitigates it.

Reviewers: zarko, #core_team, shahms

Reviewed By: #core_team, shahms

Subscribers: #core_team

Differential Revision: https://phabricator-dot-kythe-repo.appspot.com/D2039

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