Add off-thread support for class boilerplate allocation, removing a
previously "unreachable" overload. Notably, this requires support for
off-thread allocation of Dictionaries and DescriptorArrays. Due to
template fun, the off-thread allocation of Dictionaries in particular
requires some amount of boilerplate (no pun intended).

Bug: chromium:1011762
Change-Id: I37139d924858e31e45d369742329826784a8f614
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2080370
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66580}