Refactor (again) the treatment of record-selector bindings

We were generating them from the tcg_tcs field of the TcGblEnv,
but that goes badly wrong when there are top-level Template
Haskell splices, because the tcg_tcs field grows successively.
If we generate record-selector binds for all the TyCons in the
accumulated list, we generate them multiple times for TyCons
earlier in the program.  This what was happening in Trac #5665:
  data T = T { x :: Int }
  $(f 4)  -- Top level splice
  ..more code..
Here the record selector bindings for T were being generated
twice.

Better instead to generate the record-selector bindings in
TcTyClsDecls, right where the new TyCons are being declared (as indeed
they were some time ago).  This pushed me into doing some refactoring:
instead of returning the record bindings, tcTyAndClassDecls adds them
to the tcg_binds field of the TcGblEnv.  I think the result is a bit
nicer, and it has the additional merit of working.