Currently, some ScopeInfo fields are defined as indexed fields with a
length of either one or zero, because the field might be present or it
might not. Based on comments in https://crrev.com/c/v8/v8/+/2601880 ,
this strategy is not sustainable and we need a better way to represent
optional fields so that we don't have to pass zero when accessing their
only element. This change is a proposal to fix that problem.

Syntax:

I'm proposing using a question mark because TypeScript does, and Torque
syntax looks somewhat like TypeScript. I don't feel strongly about this
though, and I'm open to other suggestions.
  field_name?[condition_expression]: FieldType;

Internal Torque compiler representation:

Internally, I've updated the Torque compiler to still treat these fields
as indexed, but with an extra flag saying they're optional. When getting
a LocationReference for a field access expression on an optional field,
Torque produces a Slice like it would for any other indexed field and
subsequently calls AtIndex(0) to get a Reference.

AtIndex can crash the process if the index is out of bounds (which is
good), so some other parts of the Torque compiler need minor adjustments
so that it doesn't take references to optional fields unless it actually
needs them.

Initialization:

This proposal doesn't include any changes to initialization logic, so an
optional field can still be initialized using '...' and an iterator.
Perhaps we could introduce an Optional<T> struct for prettier
initialization in a future change.

Bug: v8:7793
Change-Id: I37649495f4c259e685261f53e4cf2859da66a31f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2706306
Commit-Queue: Seth Brenith <seth.brenith@microsoft.com>
Reviewed-by: Nico Hartmann <nicohartmann@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73018}