Custom main and global executors work hasn't passed Swift Evolution yet,
so we need to avoid leaking it as API until it does.
To that end, underscore all the things.
rdar://151147606
This is basically the same as the one for Linux, but it would be
somewhat awkward to add the platform conditional on a file named for
Linux when OpenBSD is not Linux.
Important note: if Dispatch is disabled, then this will cause a
compilation error (probably not just for OpenBSD either), because
PlatformExecutorFactory is both defined in PlatformExecutorNone.swift
and PlatformExecutor<...>.swift in this case.
Because this only bites OpenBSD bootstrap builds, and since OpenBSD
support has been upstreamed to Dispatch, default to the Dispatch
implementation for now to get this in, and we'll refactor in a different
pr.
