ScopeInfo objects generally start with three fields: flags, parameter
count, and local variable count. But a single read-only ScopeInfo
instance has none of those fields. This is the empty ScopeInfo, which is
used for contexts that don't correspond to any scope (the native context
and contexts for builtin functions). Since there is only ever a single
instance of the empty ScopeInfo, the memory savings of omitting these
fields is trivial, and we can simplify logic somewhat by including them.
Rather than checking for length to be zero, this change introduces a new
flag indicating that a ScopeInfo instance is the empty one.

On its own, this change doesn't provide a whole lot of value. However,
it sets us up for two further improvements, which are consistent with
the goals outlined in [1]:

1. We should fully describe ScopeInfo fields in Torque. Getting rid of
the requirement to check for emptiness would substantially simplify the
indexed field expressions.

2. ScopeInfo shouldn't inherit from FixedArray, and shouldn't begin with
a `length` field when the length can be computed from the other fields.
This would save a small amount of heap memory and avoid any possibility
of a mismatch between the two ways of computing the length.

[1] https://docs.google.com/document/d/1tiGK7_lubxPHnInI2vscUwMHfadn8gIEa1apmI8HxR4/edit#heading=h.n63k76b3zfwa

Bug: v8:8952
Change-Id: I018127698a5d91fb2a91684bc3aec2e27ee27c41
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2561598Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Seth Brenith <seth.brenith@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#71500}