Auto merge of #66364 - Centril:cleanup-macro-def, r=petrochenkov,eddyb

Cleanup `rmeta::MacroDef`

Avoid using rountrip parsing in the encoder and in `fn load_macro_untracked`.

The main reason I was interested in this was to remove `rustc_parse` as a dependency of `rustc_metadata` but it seems like this had other benefits as well.

Fixes #49511.

r? @eddyb
cc @matthewjasper @estebank @petrochenkov

src/librustdoc/html/render.rs

@@ -1565,7 +1565,7 @@ impl Context {
 
         let mut path = String::new();
 
-        // We can safely ignore macros from other libraries
+        // We can safely ignore synthetic `SourceFile`s.
         let file = match item.source.filename {
             FileName::Real(ref path) => path,
             _ => return None,