[DSC] Fix GetRegionAt / optimize GetRegionContaining

`GetRegionAt` was really providing the semantics of
`GetRegionContaining` due to `m_regions` being an `AddressRangeMap` and
using transparent comparators to allow `find` to work with any address
in the range.

`GetRegionAt` is updated to verify that the start address of the region
that was found matches the requested address. `GetRegionContaining` is
updated to use `AddressRangeMap::find` rather than doing a linear search
over all regions.