Auto merge of #33619 - jonathandturner:improve_structured_errors, r=nikomatsakis

Batch of improvements to errors for new error format

This is a batch of improvements to existing errors to help get the most out of the new error format.

* Added labels to primary spans (^^^) for a set of errors that didn't currently have them
* Highlight the source blue under the secondary notes for better readability
* Move some of the "Note:" into secondary spans+labels
* Fix span_label to take &mut instead, which makes it work the same as other methods in that set

src/libsyntax/errors/snippet/mod.rs

@@ -618,6 +618,7 @@ impl FileInfo {
                         styled_buffer.set_style(0, p, Style::UnderlinePrimary);
                     } else {
                         styled_buffer.putc(1, p, '-', Style::UnderlineSecondary);
+                        styled_buffer.set_style(0, p, Style::UnderlineSecondary);
                     }
                 }
             }