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- Hide GCInfoTable for most users as the table is mostly an
  implementation detail.
- Provide a type alias GCInfoIndex.
- Functional: Use relaxed memory order when reading the index in the
  fast path.  The setup is done under lock and the index retrieval
  itself should not be used for external synchronization.

Change-Id: I5f4ac9c6493520e2b719f9fc867e3337fd7b0937
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2062440
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#743165}