[pstl] Setup the _PSTL_VERSION macro like _LIBCPP_VERSION, and add release notes

Reviewers: rodgert, MikeDvorskiy

Subscribers: mgorny, jkorous, dexonsmith, libcxx-commits

Tags: #libc

Differential Revision: https://reviews.llvm.org/D60464

llvm-svn: 358193