Fix an indexing bug when binding tuple parameters with non-initial

parameter packs.

Author: rjmccall

lib/SILGen/SILGenProlog.cpp

@@ -377,7 +377,8 @@ class EmitBBArguments : public CanTypeVisitor<EmitBBArguments,
       } else {
         assert(init);
         expandPack(origEltType, substEltTypes, elt.getSubstIndex(),
-                   eltInits, elements);
+                   eltInits.slice(elt.getSubstIndex(), substEltTypes.size()),
+                   elements);
       }
     });