Figure out how to set the working directory for spawned processes on all platforms

[ahoppen]

Amazon Linux 2 and CentOS 7 have a glibc that doesn’t support posix_spawn_file_actions_addchdir_np and thus TSCBasic.Process can’t launch a process on these platforms with the working directory set. We currently fall back to launching the index tasks without a working directory on these platforms, which I think is fine because SwiftPM gives us compiler arguments with absolute paths. But we should figure something out.

Using Foundation.Process is not an option because it runs chdir on the current process for Posix platforms, which is racy if there are multiple subprocesses being spawned simultaneously. On Windows TSCBasic.Processs uses Foundation.Process and Foundation.Process properly set the working directory of the subprocesses on Windows, so Windows is not a problem.

[ahoppen]

Synced to Apple’s issue tracker as rdar://128016626

[ahoppen]

This was fixed by #1485