Tidy tyvar OccNames in TcTyClDecl

We want the universal and existential tyvars of a data constructor to
have distinct OccNames.  It's confusing if they don't (in error messages,
for example), and with the current way of generating IfaceSyn, it actally
generates bogus interface files.  (Which bit Roman.)

When IfaceSyn is full of Names, this won't matter so much, but it still
seems cleaner.

This patch adds a 'tidy' step to the generation of DataCon type
variables in TcTyClDecls.tcResultType