Add a PatternRewriter hook for cloning a region into another.

This is similar to the `inlineRegionBefore` hook, except the original blocks are unchanged. The region to be cloned *must* not have been modified during the conversion process at the point of cloning, i.e. it must belong an operation that has yet to be converted, or the operation that is currently being converted.

PiperOrigin-RevId: 273622533