conform to return-ref-scope ordering

[WalterBright]

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[dlang-bot]

Thanks for your pull request, @WalterBright!

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[WalterBright]

See #10782 for phobos fixes

[dkorpel]

Why put return under the scope instead of ref? That looks like a breaking change as return ref scope will now be translated to return scope by ReplaceTypeUnless. Although, it's not ideal either way when ParameterStorageClass doesn't know about the storage class order like the compiler does.

[WalterBright]

The design of this function is deficient because one cannot control the order of ref, return and scope. I did the best I could with it to keep it working with the unittests.

[tgehr]

return ref and return scope should be distinct parameter storage classes, like in the compiler.