Use ThreadLauncher to launch TaskPool threads

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.

Reviewers: labath, tberghammer, clayborg

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D37930

llvm-svn: 313537