[Support] Add PerThreadBumpPtrAllocator class.

PerThreadBumpPtrAllocator allows separating allocations by thread id.
That makes allocations race free. It is possible because
ThreadPoolExecutor class creates threads, keeps them until
the destructor of ThreadPoolExecutor is called, and assigns ids
to the threads. Thus PerThreadBumpPtrAllocator should be used with only
threads created by ThreadPoolExecutor. This allocator is useful when
thread safe BumpPtrAllocator is needed.

Reviewed By: MaskRay, dexonsmith, andrewng

Differential Revision: https://reviews.llvm.org/D142318