compiler: add Parser::debug_lookahead

I tried debugging a parser-related issue but found it annoying to not be
able to easily peek into the Parser's token stream.

Add a convenience fn that offers an opinionated view into the parser,
but one that is useful for answering basic questions about parser state.

compiler/rustc_parse/src/lib.rs

@@ -5,6 +5,7 @@
 #![allow(rustc::untranslatable_diagnostic)]
 #![feature(array_windows)]
 #![feature(box_patterns)]
+#![feature(debug_closure_helpers)]
 #![feature(if_let_guard)]
 #![feature(iter_intersperse)]
 #![feature(let_chains)]