This change introduces a new decorator syntax @ifdef which can be used
on any class fields in .tq files, and updates SharedFunctionInfo to use
it as an example.

Bug: v8:7793
Change-Id: I690ae2a10d6cab044eedf5b931e4f95e757ed469
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1536985
Commit-Queue: Seth Brenith <seth.brenith@microsoft.com>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61349}