Don't return complete HscEnv from upsweep

By returning a complete HscEnv from upsweep the logger (as introduced by
withDeferredDiagnostics) was escaping the scope of
withDeferredDiagnostics and hence we were losing error messages.

This is reminiscent of #20981, which also talks about writing errors
into messages after their scope has ended.

See #23305 for details.