Note [Skolemising and identity]

In some places, we make a TyVarTv for a binder. E.g.
    class C a where ...
As Note [Inferring kinds for type declarations] discusses,
we make a TyVarTv for 'a'.  Later we skolemise it, and we'd
like to retain its identity, location info etc.  (If we don't
retain its identity we'll have to do some pointless swizzling;
see GHC.Tc.TyCl.swizzleTcTyConBndrs.  If we retain its identity
but not its location we'll lose the detailed binding site info.

Conclusion: use the Name of the TyVarTv.  But we don't want
to do that when skolemising random unification variables;
there the location we want is the skolemisation site.

Fortunately we can tell the difference: random unification
variables have System Names.  That's why final_name is
set based on the isSystemName test.