Rollup merge of rust-lang#119624 - petrochenkov:dialoc4, r=compiler-errors

rustc_span: More consistent span combination operations

Also add more tests for using `tt` in addition to `ident`, and some other minor tweaks, see individual commits.

This is a part of rust-lang#119412 that doesn't yet add side tables for metavariable spans.

Author: matthiaskrgr

core/src/intrinsics/mir.rs

@@ -65,6 +65,7 @@
 //! ```rust
 //! #![feature(core_intrinsics, custom_mir)]
 //! #![allow(internal_features)]
+//! #![allow(unused_assignments)]
 //!
 //! use core::intrinsics::mir::*;
 //!