[ConstraintSolver] Attempt literal bindings earlier

If the best bindings we could get on the current step are bindings
to literal type without any other type variable involved, let's try
to attempt them right away, that should help to prune search space.

Author: xedin

lib/Sema/CSSolver.cpp

@@ -1788,14 +1788,10 @@ bool ConstraintSystem::solveSimplified(
   std::tie(bestBindings, bestTypeVar) = determineBestBindings();
 
   // If we have a binding that does not involve type variables, and is
-  // not fully bound, and is either not a literal or is a collection
-  // literal, or we have no disjunction to attempt instead, go ahead
-  // and try the bindings for this type variable.
-  if (bestBindings &&
-      (!disjunction ||
-       (!bestBindings.InvolvesTypeVariables && !bestBindings.FullyBound &&
-        (bestBindings.LiteralBinding == LiteralBindingKind::None ||
-         bestBindings.LiteralBinding == LiteralBindingKind::Collection)))) {
+  // not fully bound, or we have no disjunction to attempt instead,
+  // go ahead and try the bindings for this type variable.
+  if (bestBindings && (!disjunction || (!bestBindings.InvolvesTypeVariables &&
+                                        !bestBindings.FullyBound))) {
     return tryTypeVariableBindings(solverState->depth, bestTypeVar,
                                    bestBindings.Bindings, solutions,
                                    allowFreeTypeVariables);