...that are not composed of multiple tokens, like ("foo" "bar").  Also don't yet
warn about Boolean literals, which are sometimes wrapped in parentheses to
silence unreachable-code warnings.

To avoid multiple warnings about code like

  f((0))

switch to generally using a set of ParenExpr to keep track of which occurrences
have already been handled.

Change-Id: I036a25a92836ec6ab6c56ea848f71bc6d63822bc
Reviewed-on: https://gerrit.libreoffice.org/45317Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>