Do not check synonym RHS for ambiguity

With this patch we no longer check the RHS of a type synonym
declaration for ambiguity.  It only affects type synonyms with foralls
on the RHS (which are rare in the first place), and it's arguably
over-aggressive to check them for ambiguity.  See TcValidity
Note [When we don't check for ambiguity]

This fixes the ASSERT failures in
   th                        T3100
   typecheck/should_compile  T3692
   typecheck/should_fail     T3592