Decouple ProcessWindows from the Windows debug driver thread.

In the llgs world, ProcessWindows will eventually go away and
we'll implement a different protocol.  This patch decouples
ProcessWindows from the core debug loop so that this transition
will not be more difficult than it needs to be.

git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@221405 91177308-0d34-0410-b5e6-96231b3b80d8