Move Connection and IOObject interfaces to Utility module

Summary:
These interfaces have no dependencies, so it makes sense for them to be
in the lowest level modules, to make sure that other parts of the
codebase can use them without introducing loops.

The only exception here is the Connection::CreateDefaultConnection
method, which I've moved to Host, as it instantiates concrete
implementations, and that's where the implementations live.

Reviewers: jingham, zturner

Subscribers: lldb-commits, mgorny

Differential Revision: https://reviews.llvm.org/D34400

llvm-svn: 306391