syntax: avoid loading the same source-file multiple times

We already had a cache for file contents, but we read the source-file
before testing the cache, causing obvious slowness, so this just avoids
loading the source-file when the cache already has the contents.

src/libsyntax/codemap.rs

@@ -561,8 +561,9 @@ impl CodeMapper for CodeMap {
         sp
     }
     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)
+        file_map.add_external_src(
+            || self.file_loader.read_file(Path::new(&file_map.name)).ok()
+        )
     }
 }