C#: Address review comments

csharp/ql/src/Useless code/DefaultToString.ql

@@ -72,7 +72,7 @@ class DefaultToStringType extends ValueOrRefType {
     result instanceof SourceLocation
     or
     not super.getLocation() instanceof SourceLocation and
-    result.hasLocationInfo("", 0, 0, 0, 0)
+    result instanceof EmptyLocation
   }
 }
 

csharp/ql/src/semmle/code/csharp/ExprOrStmtParent.qll

@@ -372,7 +372,7 @@ module ExprOrStmtParentCached {
     result = min(Location l | l = e.getALocation() | l order by l.getFile().toString())
     or
     not exists(e.getALocation()) and
-    result.hasLocationInfo("", 0, 0, 0, 0)
+    result instanceof EmptyLocation
   }
 }
 private import ExprOrStmtParentCached

csharp/ql/src/semmle/code/csharp/Location.qll

@@ -51,6 +51,9 @@ class Location extends @location {
   final int getEndColumn() { this.hasLocationInfo(_, _, _, _, result) }
 }
 
+/** An empty location. */
+class EmptyLocation extends Location { EmptyLocation() { this.hasLocationInfo("", 0, 0, 0, 0) } }
+
 /**
  * A location in source code, comprising of a source file and a segment of text
  * within the file.