Rewrite the shell to be more clever so it is amenable to porting to
Windows. Use env not rather than not env to permit the lit environment
handling to take over for Windows.
...and remove the option. This is ~technically~ CLI-breaking because
Swift 5 shipped this as a hidden driver option, but it wouldn't have
/done/ anything in Swift 5, so I think it's okay to remove.
Note that if a parseable interface (.swiftinterface) and a binary
interface (.swiftmodule) are both present, the binary one will still
be preferred. This just /allows/ parseable interfaces to be used.
rdar://problem/36885834
A ‘forwarding module’ is a YAML file that’s meant to stand in for a .swiftmodule file and provide an up-to-date description of its dependencies, always using modification times.
When a ‘prebuilt module’ is first loaded, we verify that it’s up-to-date by hashing all of its dependencies. Since this is orders of magnitude slower than reading mtimes, we’ll install a `forwarding module` containing the mtimes of the now-validated dependencies.
