Store info about file "substr".

That is when a string that is part of a file needs to be parsed for a
reason, record that the string is a substr of the file rather than
using "<anon>" or "-" as the file name.  This will eventually allow
pointing to the right location, for now it just uses a more
meaningful string for the filename.

src/rustdoc/attr_parser.rs

@@ -94,7 +94,7 @@ mod test {
             mutable byte_pos: 0u
         };
         let parser = parser::new_parser_from_source_str(
-            parse_sess, [], "-", @source);
+            parse_sess, [], "-", none, @source);
 
         parser::parse_outer_attributes(parser)
     }