Auto merge of #120576 - nnethercote:merge-Diagnostic-DiagnosticBuilder, r=davidtwco

Overhaul `Diagnostic` and `DiagnosticBuilder`

Implements the first part of https://github.com/rust-lang/compiler-team/issues/722, which moves functionality and use away from `Diagnostic`, onto `DiagnosticBuilder`.

Likely follow-ups:
- Move things around, because this PR was written to minimize diff size, so some things end up in sub-optimal places. E.g. `DiagnosticBuilder` has impls in both `diagnostic.rs` and `diagnostic_builder.rs`.
- Rename `Diagnostic` as `DiagInner` and `DiagnosticBuilder` as `Diag`.

r? `@davidtwco`

compiler/rustc_expand/src/base.rs

@@ -1176,6 +1176,8 @@ impl<'a> ExtCtxt<'a> {
         for (span, notes) in self.expansions.iter() {
             let mut db = self.dcx().create_note(errors::TraceMacro { span: *span });
             for note in notes {
+                // FIXME: make this translatable
+                #[allow(rustc::untranslatable_diagnostic)]
                 db.note(note.clone());
             }
             db.emit();