Cleanup and followup to PR #17830: parsing changes

Prevents breaking down `$name` tokens into separate `$` and `name`.
Reports unknown macro variables.

Fixes #18775
Fixes #18839
Fixes #15640

src/libsyntax/parse/lexer/mod.rs

@@ -25,7 +25,7 @@ use std::rc::Rc;
 use std::str;
 use std::string::CowString;
 
-pub use ext::tt::transcribe::{TtReader, new_tt_reader};
+pub use ext::tt::transcribe::{TtReader, new_tt_reader, new_tt_reader_with_doc_flag};
 
 pub mod comments;