Summary:
This allows for the stack size to be configured, which isn't
possible with std::thread. Prevents overflowing the stack when
performing complex operations in the task pool on darwin,
where the default pthread stack size is only 512kb.
This also moves TaskPool from Utility to Host.
Reviewers: labath, tberghammer, clayborg
Subscribers: lldb-commits
Differential Revision: https://reviews.llvm.org/D37930
llvm-svn: 313637
