Rollup merge of #121783 - nnethercote:emitter-cleanups, r=oli-obk

Emitter cleanups

Some cleanups I made when reading emitter code. In particular, `HumanEmitter` and `JsonEmitter` have gone from three constructors to one.

r? `@oli-obk`

Author: GuillaumeGomez

clippy_lints/src/doc/needless_doctest_main.rs

@@ -45,7 +45,7 @@ pub fn check(
                 let fallback_bundle =
                     rustc_errors::fallback_fluent_bundle(rustc_driver::DEFAULT_LOCALE_RESOURCES.to_vec(), false);
                 let emitter = HumanEmitter::new(Box::new(io::sink()), fallback_bundle);
-                let dcx = DiagCtxt::with_emitter(Box::new(emitter)).disable_warnings();
+                let dcx = DiagCtxt::new(Box::new(emitter)).disable_warnings();
                 #[expect(clippy::arc_with_non_send_sync)] // `Lrc` is expected by with_dcx
                 let sm = Lrc::new(SourceMap::new(FilePathMapping::empty()));
                 let sess = ParseSess::with_dcx(dcx, sm);