[Type checker] Check type equality even for argument tuples in Swift 4.

Replace a where Type-pointer-equality check with what it intended,
i.e., match up ParenTypes at the top level and perform a deeper
equality comparison of the underlying types.

Fixes SR-5166 / rdar://problem/32666189.

lib/Sema/CSSimplify.cpp

@@ -1384,7 +1384,9 @@ ConstraintSystem::matchTypes(Type type1, Type type2, ConstraintKind kind,
     typeVar2 = dyn_cast<TypeVariableType>(type2.getPointer());
 
     // If the types are obviously equivalent, we're done.
-    if (type1.getPointer() == type2.getPointer())
+    if (isa<ParenType>(type1.getPointer()) ==
+          isa<ParenType>(type2.getPointer()) &&
+        type1->isEqual(type2))
       return SolutionKind::Solved;
   } else {
     typeVar1 = desugar1->getAs<TypeVariableType>();