IRGen: only apply DLLStorage if appropriate
IRGen: only apply DLLStorage if appropriate

Sinking this logic into the IRLinkage application means that we can
clean up the various sites that are handling the IRLinkage and/or
DLLStorage to apply IRLinkage semantically, making the code easier to
understand as well as avoiding the proliferation of the `useDLLStorage`
checks.

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