[backport] SI-2968 Fix brace healing for `^case (class|object) {`

Squashed commit of the following:

commit 24828531f62ce05402c96c04d7096e82d5f4e3bf
Author: Jason Zaugg <jzaugg@gmail.com>
Date:   Sun Oct 21 23:34:35 2012 +0200

    SI-2968 Fix brace healing for `^case (class|object) {`

    The scanner coalesces the pair of tokens into CASEOBJECT or
    CASECLASS, but fails to set `offset` back to the start of `case`.
    Brace healing is then unable to correctly guess the location of
    the missing brace.

    This commit resets `offset` and `lastOffset`, as though
    caseobject were a single keyword. Only the former was neccessary
    to fix this bug; I haven't found a test that shows the need for
    the latter.
    (cherry picked from commit cbad218dba47d49a39897b86d467c384538fdd53)

Review by @adriaanm