[LLDB][formatters] 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.

This functionality is tested in `TestFormatters.py`, `TestDataFormatterSynthVal.py`,
`TestDataFormatterSynthType.py`.

You may notice that the old code was passing 0 arguments when count was 1, and passing
1 argument when count is 2.

This is no longer necessary because max_positional_args counts the self pointer
correctly.

Reviewers: labath, jingham, JDevlieghere

Reviewed By: labath

Subscribers: lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D69469