[LLDB][breakpoints] ArgInfo::count -> ArgInfo::max_positional_args

Summary:
Move breakpoints from the old, bad ArgInfo::count to the new, better
ArgInfo::max_positional_args.   Soon ArgInfo::count will be no more.

It looks like this functionality is already well tested by
`TestBreakpointCommandsFromPython.py`, so there's no need to write
additional tests for it.

Reviewers: labath, jingham, JDevlieghere

Reviewed By: labath

Subscribers: lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D69468