Rollup merge of rust-lang#54374 - SergioBenitez:pub-multispan, r=alexcrichton

Make 'proc_macro::MultiSpan' public.

Oversight from rust-lang#52896.

Author: kennytm

src/libproc_macro/lib.rs

@@ -48,7 +48,7 @@ pub mod rustc;
 mod diagnostic;
 
 #[unstable(feature = "proc_macro_diagnostic", issue = "54140")]
-pub use diagnostic::{Diagnostic, Level};
+pub use diagnostic::{Diagnostic, Level, MultiSpan};
 
 use std::{ascii, fmt, iter};
 use std::path::PathBuf;