update ScriptInterpreterPython to use File, not FILE*

Summary:
ScriptInterpreterPython needs to save and restore sys.stdout and
friends when LLDB runs a python script.

It currently does this using FILE*, which is not optimal.  If
whatever was in sys.stdout can not be represented as a FILE*, then
it will not be restored correctly when the script is finished.

It also means that if the debugger's own output stream is not
representable as a file, ScriptInterpreterPython will not be able
to redirect python's  output correctly.

This patch updates ScriptInterpreterPython to represent files with
lldb_private::File, and to represent whatever the user had in
sys.stdout as simply a PythonObject.

This will make lldb interoperate better with other scripts or programs
that need to manipulate sys.stdout.

Reviewers: JDevlieghere, jasonmolenda, labath

Reviewed By: labath

Subscribers: lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D68962

llvm-svn: 374964