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Description
While working on #21332 and its predecessors, it was far too easy to store
bogus-ish values in _low or _high for unbounded ranges and to have
them come down to bite you down the line. There's no real need for these
fields when ranges are partially or completely unbounded, and a nice
housekeeping test would be to make them have the void type when not
necessary to ensure that we're not relying on their values when we shouldn't
be. This would also have the (very minor) side effect of making partially
bounded ranges more space efficient.
I didn't have the time to do this as part of #21332, so am filing it here for
future, rainy-day work.