Change representation of `Backend`

Closes #20927 (closed).

With this change, the value constructors of the Backend sum type are no longer exposed. 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. The properties and the functions that interrogate them are defined in GHC.Driver.Backend.

A record of the discussion can be found in !7442 (merged).