Move the closure hints out of environment_hints_ so that they are
handled similar to return value hints. That makes things a little
cleaner overall.

Also make it clear in terms of SLOW_DCHECKs why we don't need to
merge closure hints and, in the case of a dead environment, return
value hints.

Also rename environment_hints_ to ephemeral_hints_. The old name was
weird because these are not the only hints in the environment.

Bug: v8:7790
Change-Id: I2cb5389632c4827b027799985b8320f39e1e2314
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1643555
Auto-Submit: Georg Neis <neis@chromium.org>
Commit-Queue: Maya Lekova <mslekova@chromium.org>
Reviewed-by: Maya Lekova <mslekova@chromium.org>
Reviewed-by: Michael Stanton <mvstanton@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62019}