Fixed a crasher that has been borking out heap for a long time.

ThreadList had an assignment operator that didn't lock the "rhs" thread list object. This means a thread list can be mutated while it is being copied.

The copy constructor calls the assignment operator as well. So this fixes the unsafe threaded access to ThreadList which we believe is responsible for a lot of crashes.

<rdar://problem/28075793>

llvm-svn: 289100