[NativeProcessLinux] Remove double thread state accounting

Summary:
Now that all thread events are processed synchronously, there is no need to have separate records
of whether a thread is running. This changes the (ever-dwindling) remains of the TSC to use
NativeThreadLinux as the authoritative source of the state of threads. The rest of the
ThreadContext we need has been moved to a member of NTL.

Test Plan: ninja check-lldb continues to pass

Reviewers: chaoren, ovyalov

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D9562

llvm-svn: 236983