go/types, types2: require iterator yield to return bool (work-around)

The original implementation of the type checkers accepted any boolean
result type for yield, but the compiler's front-end had a problem with
it (#71131).

As a temporary fix (for 1.24), adjust the type checkers to insist on the
spec's literal wording and avoid the compiler panic.

Fixes #71131.
For #71164.

Change-Id: Ie25f9a892e58b5e489d399b0bce2d0af55dc3c48
Reviewed-on: https://go-review.googlesource.com/c/go/+/640599


Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Tim King <taking@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>