[NativeProcessLinux] Pass around threads by reference

Summary:
Most NPL private functions took (shared) pointers to threads as arguments. This meant that the
callee could not be sure if the pointer was valid and so most functions were peppered with
null-checks. Now, I move the check closer to the source, and pass around the threads as
references (which are then assumed to be valid).

Reviewers: tberghammer

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D12237

llvm-svn: 245831