rdar://problem/11320188

Designate MachThreadList as a transaction coordinator when doing Enable/DisableHardwareWatchpoint on the list of threads.
In case the operation (iterating on the threads and doing enable/disable) fails in the middle, we rollback the already
enabled/disabled threads to their checkpointed states.  When all the threads succeed in enable/disable, we ask each thread
to finsih the transaction and commit the change of the debug state.

llvm-svn: 157858