rustc: Correctly pretty-print macro delimiters

This commit updates the `Mac_` AST structure to keep track of the delimiters
that it originally had for its invocation. This allows us to faithfully
pretty-print macro invocations not using parentheses (e.g. `vec![...]`). This in
turn helps procedural macros due to #43081.

Closes #50840

src/libsyntax/fold.rs

@@ -520,6 +520,7 @@ pub fn noop_fold_mac<T: Folder>(Spanned {node, span}: Mac, fld: &mut T) -> Mac {
         node: Mac_ {
             tts: fld.fold_tts(node.stream()).into(),
             path: fld.fold_path(node.path),
+            delim: node.delim,
         },
         span: fld.new_span(span)
     }