To make the changes in base.tq work, there were 2 changes needed on
the C++ side:
  - calls to "FromConstexpr" are generated by the compiler for
    implicit conversions.
  - type switch is desugared and uses "Cast"

R=jgruber@chromium.org, tebbi@chromium.org

Change-Id: I085f1a393f93e501e6bbcaeacb0d6568259a4714
Reviewed-on: https://chromium-review.googlesource.com/1219629
Commit-Queue: Simon Zünd <szuend@google.com>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55794}