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.

…visions, r=lcnr

Added some `min_const_generics` revisions into `const_generics` tests

Help in rust-lang#75279. still a lot more to cover though

r? @lcnr

Remove unused function

… r=matklad

datastructures: replace `once_cell` crate with an impl from std

Fixes rust-lang#75700

r? @matklad

We might need a perf run for this change.

…y, r=matklad

Restore public visibility on some parsing functions for rustfmt

In rust-lang#74826 the visibility of several parsing functions was reduced. However, rustfmt is an external consumer of some of these functions as well and needs the visibility to be public, similar to other elements in rustc_parse such as `parse_ident`

https://github.com/rust-lang/rust/blob/db534b3ac286cf45688c3bbae6aa6e77439e52d2/src/librustc_parse/parser/mod.rs#L433-L436

rustbuild: Remove one LLD workaround

The version of LLD shipped with Rust no longer have this issue.

Closes rust-lang#68647