[llvm] [pstl] Remove PSTL from the top-level monorepo (PR #143717)

[Aaron Ballman via llvm-commits]

AaronBallman wrote:

> Ping?

For some reason the builder still isn't rebooted. IMO, this is a pretty serious gap with our infrastructure handling; we need more folks with permissions to help out in these kinds of situations.

> A trivial fix is to accept the pstl in cmake and ignore it.

I don't know CMake well enough to know how to make that work; if you know of the magic incantation to do it, I'm happy to review it so we can land it ASAP. (I'd still like to avoid a revert because of the amount of churn for something that's already fixed.)

https://github.com/llvm/llvm-project/pull/143717