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.

src/librustdoc/passes/lint/check_code_block_syntax.rs

@@ -156,7 +156,7 @@ impl Translate for BufferEmitter {
 }
 
 impl Emitter for BufferEmitter {
-    fn emit_diagnostic(&mut self, diag: &Diagnostic) {
+    fn emit_diagnostic(&mut self, diag: Diagnostic) {
         let mut buffer = self.buffer.borrow_mut();
 
         let fluent_args = to_fluent_args(diag.args());