Fix race during process detach

Summary:
The code which was preventing the usage of the OS plugin while detach is in
progress also prevented us to update the thread list correctly. This resulted
in an empty thread list, which confused the detaching logic. Change the
condition do only do what it says (disable the usage of the OS plugin).

Reviewers: clayborg, jingham

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D14201

llvm-svn: 251932