In the process, add a few simple tests for "constexpr" expressions, which
identified a few bugs that are also fixed in this CL.

Change-Id: I97486c781572642d2b574b92133b1f9cda3db592
Reviewed-on: https://chromium-review.googlesource.com/1055493
Commit-Queue: Daniel Clifford <danno@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53135}