Initialization of classes now happens atomically at the end of the
class constructor only once all of the values for the class' fields
have been fully computed. This makes Torque constructors completely
GC safe, e.g. hardened against allocations or exceptions in
constructors.

As part of this change, make the 'this' parameter for method calls
explicit rather than implicit.

Drive by: add validation to check for duplicate field declarations

Bug: v8:7793
Change-Id: I8b5e85980d6a103ef9fc3262b76f6514f36ebf88
Reviewed-on: https://chromium-review.googlesource.com/c/1411252
Commit-Queue: Daniel Clifford <danno@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58979}