This aliasing makes subsequent commits easier to review; eventually CoinsTip()
will return the CCoinsViewCache managed by CChainState.
https://github.com/bitcoin/bitcoin/pull/16443/commits/fae6ab6aed3b9fdc9201bb19a307dfc3d9b89891
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Depends on D7578 and D7579
Partial backport of Core [[https://github.com/bitcoin/bitcoin/pull/16443 | PR16443]]