remove File::SetStream(), make new files instead.

Summary:
This patch removes File::SetStream() and File::SetDescriptor(),
and replaces most direct uses of File with pointers to File.
Instead of calling SetStream() on a file, we make a new file and
replace it.

My ultimate goal here is to introduce a new API class SBFile, which
has full support for python io.IOStream file objects.   These can
redirect read() and write() to python code, so lldb::Files will
need a way to dispatch those methods.   Additionally it will need some
form of sharing and assigning files, as a SBFile will be passed in and
assigned to the main IO streams of the debugger.

In my prototype patch queue, I make File itself copyable and add a
secondary class FileOps to manage the sharing and dispatch.  In that
case SBFile was a unique_ptr<File>.
(here: https://github.com/smoofra/llvm-project/tree/files)

However in review, Pavel Labath suggested that it be shared_ptr instead.
(here: https://reviews.llvm.org/D67793)

In order for SBFile to use shared_ptr<File>, everything else should
as well.

If this patch is accepted, I will make SBFile use a shared_ptr
I will remove FileOps from future patches and use subclasses of File
instead.

Reviewers: JDevlieghere, jasonmolenda, zturner, jingham, labath

Reviewed By: labath

Subscribers: lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D67891

llvm-svn: 373090