parse_format: treat r" as a literal
This PR changes `format_args!` internal parsing machinery to treat raw strings starting `r"` as a literal.
Currently `"` and `r#` are recognised as valid starting combinations for string literals, but `r"` is not.
This was noticed when debugging https://github.com/rust-lang/rust/issues/67984#issuecomment-753413156
As well as fixing the behavior observed in that comment, this improves diagnostic spans for `r"` formatting strings.