Add a TaskQueue that can serialize work on a ThreadPool.

We have ThreadPool, which can execute work asynchronously on N
background threads, but sometimes you need to make sure the work
is executed asynchronously but also serially.  That is, if task
B is enqueued after task A, then task B should not begin until
task A has completed.  This patch adds such a class.

Differential Revision: https://reviews.llvm.org/D48240

llvm-svn: 335440