While the class Type objects the AstTyper generates should be
allocated to zone memory, there's no particular reason
the typer object itself needs to be.

BUG=None
TEST=None
R=rossberg@chromium.org,titzer@chromium.org
LOG=N

Review URL: https://codereview.chromium.org/1303843003

Cr-Commit-Position: refs/heads/master@{#30313}