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
Fix a typo that caused us to not include the correct code for platforms
that use "singlethreaded" concurrency.
Also use `var` not `let` for the computed property in `PlatformExecutorFactory`.
rdar://141348916