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

include/swift/AST/KnownIdentifiers.def

@@ -35,6 +35,7 @@ IDENTIFIER(Any)
 IDENTIFIER(ArrayLiteralElement)
 IDENTIFIER(asLocalActor)
 IDENTIFIER(atIndexedSubscript)
+IDENTIFIER(basic_string)
 IDENTIFIER_(bridgeToObjectiveC)
 IDENTIFIER(buildArray)
 IDENTIFIER(buildBlock)

include/swift/AST/Types.h

@@ -1112,6 +1112,9 @@ public:
   /// Check if this is a ObjCBool type from the Objective-C module.
   bool isObjCBool();
 
+  /// Check if this is a std.string type from C++.
+  bool isCxxString();
+
   /// Check if this is the type Unicode.Scalar from the Swift standard library.
   bool isUnicodeScalar();