[CSOptimizer] Rank operators with the same score based on speculative status

If the scores are the same and both disjunctions are operators
they could be ranked purely based on whether the candidates
were speculative or not. The one with more context always wins.

Consider the following situation:

```swift
func test(_: Int) { ... }
func test(_: String) { ... }

test("a" + "b" + "c")
```

In this case we should always prefer ... + "c" over "a" + "b"
because it would fail and prune the other overload if parameter
type (aka contextual type) is `Int`.

Author: xedin

lib/Sema/CSOptimizer.cpp

@@ -1846,6 +1846,24 @@ ConstraintSystem::selectDisjunction() {
           // based on number of favored/active choices.
           if (*firstScore != *secondScore)
             return *firstScore > *secondScore;
+
+          // If the scores are the same and both disjunctions are operators
+          // they could be ranked purely based on whether the candidates
+          // were speculative or not. The one with more context always wins.
+          //
+          // Consider the following situation:
+          //
+          // func test(_: Int) { ... }
+          // func test(_: String) { ... }
+          //
+          // test("a" + "b" + "c")
+          //
+          // In this case we should always prefer ... + "c" over "a" + "b"
+          // because it would fail and prune the other overload if parameter
+          // type (aka contextual type) is `Int`.
+          if (isFirstOperator && isSecondOperator &&
+              isFirstSpeculative != isSecondSpeculative)
+            return isSecondSpeculative;
         }
 
         // Use favored choices only if disjunction score is higher