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_ext/assert.rs

@@ -53,6 +53,7 @@ pub fn expand_assert<'cx>(
                 ),
             )).into()
         },
+        delim: MacDelimiter::Parenthesis,
     };
     let if_expr = cx.expr_if(
         sp,