This removes all but one caller of Literal::raw_value(), thus
hiding AstValue from the rest of the codebase. This is in
preparation to move much of AstValue's implementation up
into Literal itself, thus avoiding the overhead of the
underling ZoneObjects and allowing us to remove complexity
such as the cache of Smi-valued AstValues.

Bug: v8:6984
Change-Id: I1b90aa64b9d26db36ef486afe73cda4473ef866e
Reviewed-on: https://chromium-review.googlesource.com/731109Reviewed-by: Marja Hölttä <marja@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48884}