Norman Ramsey authored Feb 07, 2022 and Cheng Shao committed May 21, 2022

With this change, `Backend` becomes an abstract type
(there are no more exposed value constructors).
Decisions that were formerly made by asking "is the
current back end equal to (or different from) this named value
constructor?" are now made by interrogating the back end about
its properties, which are functions exported by `GHC.Driver.Backend`.

There is a description of how to migrate code using `Backend` in the
user guide.

Clients using the GHC API can find a backdoor to access the Backend
datatype in GHC.Driver.Backend.Internal.

Bumps haddock submodule.

Fixes #20927