Fixes#81007
Previously, we would fail to collect tokens in the proper place when
only builtin attributes were present. As a result, we would end up with
attribute tokens in the collected `TokenStream`, leading to duplication
when we attempted to prepend the attributes from the AST node.
We now explicitly track when token collection must be performed due to
nomterminal parsing.
StringReader is an intornal abstraction which at the moment changes a
lot, so these unit tests cause quite a bit of friction.
Moving them to rustc_lexer and more ingerated-testing style should
make them much less annoying, hopefully without decreasing their
usefulness much.
Note that coloncolon tests are removed (it's unclear what those are
testing).
\r\n tests are removed as well, as we normalize line endings even
before lexing.
