Initial PSTL commit

The initial commit of the Parallel STL upstream (under LLVM umbrella) based on
Parallel STL 20181204 open source release, which is available by
https://github.com/intel/parallelstl

Author: Mikhail Dvorskiy <mikhail.dvorskiy@intel.com>

Differential Revision: https://reviews.llvm.org/D55889

llvm-svn: 349653