- Don't pass 'verify' since it's now the default
- Update tests where diagnostics changed in a correct way to pass 'on' instead
- Delete compiler_scale/explicit_requirements_perf.swift since it's not testing anything with the requirement machine
When emitting a diagnostic, mark the TypeRepr as invalid and
return an ErrorType to ensure that the diagnostic is not
emitted again, and to muffle downstream diagnostics.
