If the data to be stored in a BitVector is small enough (less than the
pointer size), it can be stored directly on the BitVector instead of the
pointer. This patch makes the data field of the BitVector a union
between a pointer and uintptr_t, and uses the latter is the data length
is 0.

Change-Id: I24c1920f2c16373c883cf69b123bf59812fef28e
Reviewed-on: https://chromium-review.googlesource.com/541307
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46209}