Re-land "get rid of PythonInteger::GetInteger()"

This was reverted due to a python2-specific bug.  Re-landing with a fix
for python2.

Summary:
One small step in my long running quest to improve python exception handling in
LLDB.  Replace GetInteger() which just returns an int with As<long long> and
friends, which return Expected types that can track python exceptions

Reviewers: labath, jasonmolenda, JDevlieghere, vadimcn, omjavaid

Reviewed By: labath, omjavaid

Subscribers: omjavaid, lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D78462