SBFile::GetFile: convert SBFile back into python native files.

Summary:
This makes SBFile::GetFile public and adds a SWIG typemap to convert
the result back into a python native file.

If the underlying File itself came from a python file, it is returned
identically. Otherwise a new python file object is created using
the file descriptor.

Reviewers: JDevlieghere, jasonmolenda, labath

Reviewed By: labath

Subscribers: lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D68737