[ConstraintSystem] Change the order in which we attempt disjunctions

to be stable.

We currently will stop visiting the elements of a disjunction under
certain circumstances once we have found a solution. The result we get
is inherently dependent on the order in which we determine to visit
the disjunctions themselves (in addition to the elements of the
disjunction).

This change makes the order in which we visit disjunctions
stable. Future commits will create a stable ordering for the elements
of disjunctions. Once we also have that stable ordering in place we
can in theory short circuit more often as part of changing the way in
which we decide what the "best" solution is to a system.

This results in an expression in
validation-test/stdlib/AnyHashable.swift.gyb no longer being able to
typecheck in a reasonable amount of time, so I had to tweak that
expression.

lib/Sema/Constraint.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
@@ -798,6 +798,7 @@ Constraint *Constraint::createDisjunction(ConstraintSystem &cs,
   auto disjunction =  new (mem) Constraint(ConstraintKind::Disjunction,
                               cs.allocateCopy(constraints), locator, typeVars);
   disjunction->RememberChoice = (bool) rememberChoice;
+  cs.noteNewDisjunction(disjunction);
   return disjunction;
 }