Combine GREs when combining in mkImportOccEnv

In `GHC.Rename.Names.mkImportOccEnv`, we sometimes discard one import
item in favour of another, as explained in Note [Dealing with imports]
in `GHC.Rename.Names`. However, this can cause us to lose track of
important parent information.

Consider for example #24084:

  module M1 where { class C a where { type T a } }
  module M2 ( module M1 ) where { import M1 }
  module M3 where { import M2 ( C, T ); instance C () where T () = () }

When processing the import list of `M3`, we start off (for reasons that
are not relevant right now) with two `Avail`s attached to `T`, namely
`C(C, T)` and `T(T)`. We combine them in the `combine` function of
`mkImportOccEnv`; as described in Note [Dealing with imports] we discard
`C(C, T)` in favour of `T(T)`. However, in doing so, we **must not**
discard the information want that `C` is the parent of `T`. Indeed,
losing track of this information can cause errors when importing,
as we could get an error of the form

  ‘T’ is not a (visible) associated type of class ‘C’

We fix this by combining the two GREs for `T` using `plusGRE`.

Fixes #24084