My previous patch to add DISubrangeType (#126772) had a couple of minor
errors. This patch corrects them.
1. When using a DISubrangeType as an array index type, the wrong tag was
written into the DIE.
2. I'd intended for subranges to use bit strides, not byte strides --
but neglected to actually implement this. Ada needs bit strides.
This patch adds a new test that checks both these things.
Finally, this patch adds some documentation for DISubrangeType.
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