Make Emitter::emit_diagnostic consuming.

All the other `emit`/`emit_diagnostic` methods were recently made
consuming (e.g. #119606), but this one wasn't. But it makes sense to.

Much of this is straightforward, and lots of `clone` calls are avoided.
There are a couple of tricky bits.
- `Emitter::primary_span_formatted` no longer takes a `Diagnostic` and
  returns a pair. Instead it takes the two fields from `Diagnostic` that
  it used (`span` and `suggestions`) as `&mut`, and modifies them. This
  is necessary to avoid the cloning of `diag.children` in two emitters.
- `from_errors_diagnostic` is rearranged so various uses of `diag` occur
  before the consuming `emit_diagnostic` call.

compiler/rustc_codegen_ssa/src/back/write.rs

@@ -1810,7 +1810,7 @@ fn fallback_fluent_bundle(&self) -> &FluentBundle {
 }
 
 impl Emitter for SharedEmitter {
-    fn emit_diagnostic(&mut self, diag: &rustc_errors::Diagnostic) {
+    fn emit_diagnostic(&mut self, diag: rustc_errors::Diagnostic) {
         let args: FxHashMap<DiagnosticArgName, DiagnosticArgValue> =
             diag.args().map(|(name, arg)| (name.clone(), arg.clone())).collect();
         drop(self.sender.send(SharedEmitterMessage::Diagnostic(Diagnostic {