Fix estimate_num_groups() to not fail on PlaceHolderVars, per report from

Fix estimate_num_groups() to not fail on PlaceHolderVars, per report from
Stefan Kaltenbrunner.  The most reasonable behavior (at least for the near
term) seems to be to ignore the PlaceHolderVar and examine its argument
instead.  In support of this, change the API of pull_var_clause() to allow
callers to request recursion into PlaceHolderVars.  Currently
estimate_num_groups() is the only customer for that behavior, but where
there's one there may be others.