allow arbitrary python streams to be converted to SBFile

Summary:
This patch adds SWIG typemaps that can convert arbitrary python
file objects into lldb_private::File.

A SBFile may be initialized from a python file using the
constructor.   There are also alternate, tagged constructors
that allow python files to be borrowed, and for the caller
to control whether or not the python I/O methods will be
called even when a file descriptor is available.I

Reviewers: JDevlieghere, jasonmolenda, labath

Reviewed By: labath

Subscribers: zturner, amccarth, lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D68188

llvm-svn: 374225