[pstl] Initial integration with LLVM's CMake

Summary:
This commit adds a check-pstl CMake target that will run the tests
we currently have for pstl. Those tests are not using LLVM lit yet,
but switching them over should be a transparent change. With this
change, we can start relying on the `check-pstl` target for workflows
and CI.

Note that this commit purposefully does not support the pre-monorepo
layout (with subprojects in projects/), since LLVM is moving towards
the monorepo layout anyway.

Reviewers: jfb

Subscribers: mgorny, jkorous, dexonsmith, libcxx-commits, mclow.lists, rodgert

Differential Revision: https://reviews.llvm.org/D55963

llvm-svn: 349919