They need to be properly recorded in the scope's temps set, otherwise allocation doesn't know about them and can break. (Not observable right now, but necessary for follow-up changes to parameter destructuring.)

Also, print temporary variables in a useful manner.

R=adamk@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1263563002

Cr-Commit-Position: refs/heads/master@{#29998}