Revert incorrect change to type of Index::table

which is thankfully mostly latent since it's only used directly
by dump_cache. Add a comment explaining what the heck is going on.
Also update dump_cache to the safer approach rather than UNSAFE_TODO
and likely-undefined-behavior crash on corrupt data.

Change-Id: I7135667faa7a6ca2e7e562ad0dcc41268a899f10
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6792823
Reviewed-by: Stephen Nusko <nuskos@chromium.org>
Reviewed-by: Adam Rice <ricea@chromium.org>
Commit-Queue: Maks Orlovich <morlovich@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1494127}