[ConstraintSystem] Extract "best disjunction" selection mechanism into a separate method

Author: xedin

include/swift/Sema/ConstraintSystem.h

@@ -5033,6 +5033,9 @@ class ConstraintSystem {
   ///
   /// \returns The selected disjunction.
   Constraint *selectDisjunction();
+  /// Select the best possible disjunction for solver to attempt
+  /// based on the given list.
+  Constraint *selectBestDisjunction(ArrayRef<Constraint *> disjunctions);
 
   /// Pick a conjunction from the InactiveConstraints list.
   ///

lib/Sema/CSSolver.cpp

@@ -1662,8 +1662,9 @@ ConstraintSystem::filterDisjunction(
 // right-hand side of a conversion constraint, since having a concrete
 // type that we're converting to can make it possible to split the
 // constraint system into multiple ones.
-static Constraint *selectBestBindingDisjunction(
-    ConstraintSystem &cs, SmallVectorImpl<Constraint *> &disjunctions) {
+static Constraint *
+selectBestBindingDisjunction(ConstraintSystem &cs,
+                             ArrayRef<Constraint *> disjunctions) {
 
   if (disjunctions.empty())
     return nullptr;
@@ -2170,6 +2171,13 @@ Constraint *ConstraintSystem::selectDisjunction() {
   if (disjunctions.empty())
     return nullptr;
 
+  return selectBestDisjunction(disjunctions);
+}
+
+Constraint *
+ConstraintSystem::selectBestDisjunction(ArrayRef<Constraint *> disjunctions) {
+  assert(!disjunctions.empty());
+
   if (auto *disjunction = selectBestBindingDisjunction(*this, disjunctions))
     return disjunction;