This is a partial reland of https://crrev.com/c/2601880 .

In preparation for ScopeInfo not being a FixedArrayBase, this change
privatizes the FixedArray-style functions that provide access to
ScopeInfo fields by index, and moves them from scope-info-inl.h to
scope-info.cc. Those functions are still used pretty heavily during
initialization (ScopeInfo::Create, etc.), but at least we can avoid
presenting them to the rest of the world.

This change also introduces a new length() function in ScopeInfo which
hides the one inherited from FixedArrayBase and computes the ScopeInfo's
length based on its flags, so that there are no remaining readers of the
'length' field.

Change-Id: I609754010723b679e5cf00f386020faaab84c17a
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2718275
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73151}