Get a decent MatchContext for pattern synonym bindings

In particular when we have a pattern binding
    K p1 .. pn = rhs
where K is a pattern synonym.  (It might be nested.)

This small MR fixes #25995.  It's a tiny fix, to an error message,
removing an always-dubious `unkSkol`.

The bug report was in the context of horde-ad, a big program,
and I didn't manage to make a small repro case quickly. I decided
not to bother further.

compiler/GHC/Tc/Gen/Pat.hs

@@ -1265,9 +1265,10 @@ tcPatSynPat (L con_span con_name) pat_syn pat_ty penv arg_pats thing_inside
         ; let all_arg_tys = ty : prov_theta ++ (map scaledThing arg_tys)
         ; checkGADT (PatSynCon pat_syn) ex_tvs all_arg_tys penv
 
-        ; skol_info <- case pe_ctxt penv of
-                            LamPat mc -> mkSkolemInfo (PatSkol (PatSynCon pat_syn) mc)
-                            LetPat {} -> return unkSkol -- Doesn't matter
+        ; let match_ctxt = case pe_ctxt penv of
+                            LamPat mc -> mc
+                            LetPat {} -> PatBindRhs
+        ; skol_info <- mkSkolemInfo (PatSkol (PatSynCon pat_syn) match_ctxt)
 
         ; (tenv, ex_tvs') <- tcInstSuperSkolTyVarsX skol_info subst ex_tvs
            -- This freshens: Note [Freshen existentials]