Introduce a proper empty_descriptor_array, which has the proper layout
(length is 2 and the two fields are set properly). Also add a special
EnumCache class and a matching empty_enum_cache. The contract now is
that we only need to check the EnumLength on the map to know whether we
are allowed to use the enum cache. This greatly simplifies the handling
of the enum cache (and also the descriptor arrays), especially for the
future work on optimizing keyed access via the enum cache indices.

Bug: v8:6702
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
Change-Id: I5ef517a3041163cd65ef003f691139ea52233e83
Reviewed-on: https://chromium-review.googlesource.com/641030
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47697}