This code is public on main, but not on 6.2, so if one compiles a binary for 6.2
on main and then attempts to use a stdlib from a 6.2 aligned macOS, one gets an
ABI error since the type isn't there. Just mark these types as being available
in 6.3 so that way this is future proofed and allows for main binaries to run
against a 6.2 aligned macOS stdlib.
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
