[ThreadPool] Support returning futures with results.

This patch adjusts ThreadPool::async to return futures that wrap
the result type of the passed in callable.

To do so, ThreadPool::asyncImpl first creates a shared promise. The
result of the promise is set in a new callable that first executes the
task. The callable is added to the task queue.

Reviewed By: mehdi_amini

Differential Revision: https://reviews.llvm.org/D114183