Add a frontend option to disable the "shrink" part of the solver.

We sometimes see expression type checking times increase dramatically
when this is enabled, and having a way to disable will make it
possible to easily do measurements to determine the cost/benefit of
having this enabled.

lib/Frontend/CompilerInvocation.cpp

@@ -2,7 +2,7 @@
 //
 // This source file is part of the Swift.org open source project
 //
-// Copyright (c) 2014 - 2017 Apple Inc. and the Swift project authors
+// Copyright (c) 2014 - 2018 Apple Inc. and the Swift project authors
 // Licensed under Apache License v2.0 with Runtime Library Exception
 //
 // See https://swift.org/LICENSE.txt for license information
@@ -279,6 +279,9 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
     Opts.SolverShrinkUnsolvedThreshold = threshold;
   }
 
+  if (const Arg *A = Args.getLastArg(OPT_solver_disable_shrink))
+    Opts.SolverDisableShrink = true;
+
   if (const Arg *A = Args.getLastArg(OPT_value_recursion_threshold)) {
     unsigned threshold;
     if (StringRef(A->getValue()).getAsInteger(10, threshold)) {