factor out an abstract base class for File

Summary:
This patch factors out File as an abstract base
class and moves most of its actual functionality into
a subclass called NativeFile.   In the next patch,
I'm going to be adding subclasses of File that
don't necessarily have any connection to actual OS files,
so they will not inherit from NativeFile.

This patch was split out as a prerequisite for
https://reviews.llvm.org/D68188

Reviewers: JDevlieghere, jasonmolenda, labath

Reviewed By: labath

Subscribers: lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D68317

llvm-svn: 373564