The base class llvm::ThreadPoolInterface will be renamed
llvm::ThreadPool in a subsequent commit.
This is a breaking change: clients who use to create a ThreadPool must
now create a DefaultThreadPool instead.
LazyAtomicPointer is a lock-free pointer that can coordinate concurrent
writes to a pointer using a generator.
Reviewed By: benlangmuir
Differential Revision: https://reviews.llvm.org/D133714