[lldb] Construct the dummy target when the first Debugger object is constructed

Summary:
We should always have a dummy target, so we might as well construct it directly when we create a Debugger object.

The idea is that if this patch doesn't cause any problems that we can get rid of all the logic
that handles situations where we don't have a dummy target (as all that code is currently
untested as there seems to be no way to have no dummy target in LLDB).

Reviewers: labath, jingham

Reviewed By: labath, jingham

Subscribers: jingham, abidh, lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D66581

llvm-svn: 369885