[ConstraintSystem] Fail transitive conformance constraints without checking

implicit conversions if the type does not conform and the current score
is equal to the best score.

Implicit conversions will always increase the score, so if the current
score is equal to the best score and an implicit conversion is needed to
satisfy a conformance requirement, the solver would end up pruning that
path later on anyway. It's better to fail early.

Author: hborla

lib/Sema/CSSimplify.cpp

@@ -6479,6 +6479,12 @@ ConstraintSystem::SolutionKind ConstraintSystem::simplifyTransitivelyConformsTo(
   // If the type doesn't conform, let's check whether
   // an Optional or Unsafe{Mutable}Pointer from it would.
 
+  // If the current score is equal to the best score, fail without checking
+  // implicit conversions, because an implicit conversion would lead to a
+  // worse score anyway.
+  if (solverState && solverState->BestScore && CurrentScore == *solverState->BestScore)
+    return SolutionKind::Error;
+
   SmallVector<Type, 4> typesToCheck;
 
   // T -> Optional<T>