Ensure that users cannot use generated arguments.

This commit gensyms the generated ident for replacement arguments so
that users cannot refer to them. It also ensures that levenshtein
distance suggestions do not suggest gensymed identifiers.

src/libsyntax/parse/parser.rs

@@ -8878,7 +8878,7 @@ impl<'a> Parser<'a> {
 
                 // Construct a name for our temporary argument.
                 let name = format!("__arg{}", index);
-                let ident = Ident::from_str(&name);
+                let ident = Ident::from_str(&name).gensym();
 
                 // Check if this is a ident pattern, if so, we can optimize and avoid adding a
                 // `let <pat> = __argN;` statement, instead just adding a `let <pat> = <pat>;`