Fix Trac #4127 (and hence #4173)

Fix Trac #4127 (and hence #4173)

The change involves a little refactoring, so that the default
method Ids are brought into scope earlier, before the value
declarations are compiled.  (Since a value decl may contain
an instance decl in a quote.)

See Note [Default method Ids and Template Haskell] in
TcTyClsDcls.