Create a ConnectionGenericFile class for Windows.

This is the first step in getting ConnectionFileDescriptor ported
to Windows. It implements a connection against a disk file for
windows. This supports connection strings of the form file://PATH
which are currently supported only on posix platforms in
ConnectionFileDescriptor.

Reviewed by: Greg Clayton
Differential Revision: http://reviews.llvm.org/D5608

llvm-svn: 219145