more compact encoding of relativeLoc
more compact encoding of relativeLoc

Summary:
<!--
  If this is a change to library defintions, please include links to relevant documentation.
  If this is a documentation change, please prefix the title with [DOCS].

  If this is neither, ensure you opened a discussion issue and link it in the PR description.
-->
This is part of the work of https://github.com/facebook/flow/issues/8739
It gives a more compact encoding when the Loc shares the same line which happens a lot

Changelog: [perf] Smaller memory use thanks to more compact encoding of relative locations

Pull Request resolved: https://github.com/facebook/flow/pull/8746

Reviewed By: mroch

Differential Revision: D30953183

Pulled By: panagosg7

fbshipit-source-id: 2589c81859f09c8f11ebbb7e6c8d64d2fabab11d

facebook/flow,mroch/flow,mroch/flow,facebook/flow,mroch/flow,facebook/flow,facebook/flow,facebook/flow,facebook/flow,mroch/flow,mroch/flow,mroch/flow,facebook/flow,mroch/flow