Don't allocate AstTyper with the zone allocator.

Don't allocate AstTyper with the zone allocator.

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}