format_args! is broken

[Veykril]

          The `format_args` is broken even on nightly. The problem is that `Argument` is defined inside `core::fmt::rt` which is a private module so even if we use `core::fmt::rt::Argument` (we are currently using `core::fmt::Argument`) it wouldn't work. So it seems lang paths are the only way to access that. We need to either make the lang paths a special ast node, or make the entire `format_args!()` a special ast node similar to what rustc does.

Originally posted by @HKalbasi in #14980 (comment)

We'll need to think of a way to address this in a future proofed way as this issue may re-appear whether it be for format_args or another new macro