[pstl] Add a serial backend for the PSTL

Summary:
The serial backend performs all tasks serially and does not require
threads. It does not have any dependencies beyond normal C++, but
it is not very efficient either.

Reviewers: rodgert, MikeDvorskiy

Subscribers: mgorny, jkorous, dexonsmith, jdoerfert, libcxx-commits

Differential Revision: https://reviews.llvm.org/D59791

llvm-svn: 358700