Report `kRegexpBadPerlOp` for look-behind assertions.

Fixes #468.

Change-Id: I9a72db0bbb9a04e5081cc1f0f94399476b63d1c3
Reviewed-on: https://code-review.googlesource.com/c/re2/+/62330
Reviewed-by: Perry Lorier <perryl@google.com>
Reviewed-by: Paul Wankadia <junyer@google.com>
2 files changed