External spans: address review.

* The lazy loading mechanism has been moved to a more appropriate place.
* Return values from the functions invoked there are properly used.
* Documentation has gotten some minor improvements.
* Possibly some larger restructuring will need to take place still.

src/libsyntax/codemap.rs

@@ -559,19 +559,9 @@ impl CodeMapper for CodeMap {
         }
         sp
     }
-    fn load_source_for_filemap(&self, filename: FileName) -> bool {
-        let file_map = if let Some(fm) = self.get_filemap(&filename) {
-            fm
-        } else {
-            return false;
-        };
-
-        if *file_map.external_src.borrow() == ExternalSource::AbsentOk {
-            let src = self.file_loader.read_file(Path::new(&filename)).ok();
-            return file_map.add_external_src(src);
-        }
-
-        false
+    fn ensure_filemap_source_present(&self, file_map: Rc<FileMap>) -> bool {
+        let src = self.file_loader.read_file(Path::new(&file_map.name)).ok();
+        return file_map.add_external_src(src)
     }
 }