convert LLDBSwigPythonCallTypeScript to ArgInfo::max_positional_args

Summary:
This patch converts another user of ArgInfo::count over to
use ArgInfo::max_positional_args instead.   I also add a test
to make sure both documented signatures for python type formatters
work.

Reviewers: JDevlieghere, clayborg, labath, jingham

Reviewed By: labath

Subscribers: lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D69153

llvm-svn: 375334