Rollup merge of #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/rust#119412 that doesn't yet add side tables for metavariable spans.

Author: matthiaskrgr