This CL adds 'typeswitch' and 'case' as a keyword and removes some
unused ones. It also adds 'UncheckedCast', 'Cast', 'Convert' and
'FromConstexpr' as support functions.

Bug: v8:7793
Change-Id: Ia58045a5fb0f599c7c17cbf11d4e60ed55db7334
Reviewed-on: https://chromium-review.googlesource.com/1219389Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Simon Zünd <szuend@google.com>
Cr-Commit-Position: refs/heads/master@{#55850}