new api class: SBFile

Summary:
SBFile is a scripting API wrapper for lldb_private::File

This is the first step in a project to enable arbitrary python
io.IOBase file objects -- including those that override the read()
and write() methods -- to be used as the main debugger IOStreams.

Currently this is impossible because python file objects must first
be converted into FILE* streams by SWIG in order to be passed into
the debugger.

full prototype: https://github.com/smoofra/llvm-project/tree/files

Reviewers: JDevlieghere, jasonmolenda, zturner, jingham, labath

Reviewed By: labath

Subscribers: labath, mgorny, lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D67793

llvm-svn: 373562