Add a new task pool class to LLDB

The purpose of the class is to make it easy to execute tasks in parallel

Basic design goals:
* Have a very lightweight and easy to use interface where a list of
  lambdas can be executed in parallel
* Use a global thread pool to limit the number of threads used
  (std::async don't do it on Linux) and to eliminate the thread creation
  overhead
* Destroy the thread currently not in use to avoid the confusion caused
  by them during debugging LLDB

Possible future improvements:
* Possibility to cancel already added, but not yet started tasks
* Parallel for_each implementation
* Optimizations in the thread creation destroyation code

Differential revision: http://reviews.llvm.org/D13727

llvm-svn: 250820