Add Bounded DynFlags.Language

[ndmitchell]

Would be great if this had a Bounded instance. Probably best upstream?

[shayne-fletcher]

Ticket raised https://gitlab.haskell.org/ghc/ghc/merge_requests/2905.

[shayne-fletcher]

Landed. Available from ghc-8.12.1.