template-haskell: define `MonadFail Q` instance

When `MonadFail`is available, this patch makes `MonadFail` a superclass
of `Quasi`, and `Q` an instance of `MonadFail`.

NB: Since f16ddcee, we need to be able
    to compile `template-haskell` with stage0 compilers that don't provide
    a `MonadFail` class yet. Once we reach GHC 8.3 development we can drop
    the CPP conditionals again.

Addresses #11661

Reviewed By: bgamari, goldfire

Differential Revision: https://phabricator.haskell.org/D1982