Reapply "[ConstraintSystem] C++ Interop: Binding a string literal to std.string shouldn't increase the score"

This reverts commit 6852bc9834.

In addition to the original change, this makes sure that C++ `std::string` and Swift `String` are given distinct score, in order to prevent ambiguity which was causing build failures in some projects.

rdar://158439395

lib/Sema/ConstraintSystem.cpp

@@ -257,10 +257,24 @@ void ConstraintSystem::assignFixedType(TypeVariableType *typeVar, Type type,
 
       // If the protocol has a default type, check it.
       if (auto defaultType = TypeChecker::getDefaultType(literalProtocol, DC)) {
-        // Check whether the nominal types match. This makes sure that we
-        // properly handle Array vs. Array<T>.
-        if (defaultType->getAnyNominal() != type->getAnyNominal()) {
-          increaseScore(SK_NonDefaultLiteral, locator);
+        auto isDefaultType = [&defaultType](Type type) {
+          // Check whether the nominal types match. This makes sure that we
+          // properly handle Array vs. Array<T>.
+          return defaultType->getAnyNominal() == type->getAnyNominal();
+        };
+
+        if (!isDefaultType(type)) {
+          // Treat `std.string` as a default type just like we do
+          // Swift standard library `String`. This helps to disambiguate
+          // operator overloads that use `std.string` vs. a custom C++
+          // type that conforms to `ExpressibleByStringLiteral` as well.
+          bool isCxxDefaultType =
+              literalProtocol->isSpecificProtocol(
+                  KnownProtocolKind::ExpressibleByStringLiteral) &&
+              type->isCxxString();
+
+          increaseScore(SK_NonDefaultLiteral, locator,
+                        isCxxDefaultType ? 1 : 2);
         }
       }