Rename some Diagnostic setters.

`Diagnostic` has 40 methods that return `&mut Self` and could be
considered setters. Four of them have a `set_` prefix. This doesn't seem
necessary for a type that implements the builder pattern. This commit
removes the `set_` prefixes on those four methods.

Author: nnethercote

src/errors.rs

@@ -119,12 +119,12 @@ impl<G: EmissionGuarantee> IntoDiagnostic<'_, G> for TargetFeatureDisableOrEnabl
             fluent::codegen_gcc_target_feature_disable_or_enable
         );
         if let Some(span) = self.span {
-            diag.set_span(span);
+            diag.span(span);
         };
         if let Some(missing_features) = self.missing_features {
             diag.subdiagnostic(missing_features);
         }
-        diag.set_arg("features", self.features.join(", "));
+        diag.arg("features", self.features.join(", "));
         diag
     }
 }