incremental: migrate diagnostics
- Apply the diagnostic migration lints to more functions on `Session`, namely: `span_warn`, `span_warn_with_code`, `warn` `note_without_error`, `span_note_without_error`, `struct_note_without_error`.
- Add impls of `IntoDiagnosticArg` for `std::io::Error`, `std::path::Path` and `std::path::PathBuf`.
- Migrate the `rustc_incremental` crate's diagnostics to translatable diagnostic structs.
r? `@compiler-errors`
cc #100717
a64ef7d07d