Rename ToNumber to PlainPrimitiveToNumber since it must only be used on
primitives, as we assume that it never throws and has no side effects.

Change-Id: I78880545e58e46d38712f5ab75fe0b627ad178c5
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2002394
Auto-Submit: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65796}