Change span of as_dyn_error() to point compile error at attribute.

tests/ui/source-enum-unnamed-field-not-error.rs

@@ -0,0 +1,12 @@
+use thiserror::Error;
+
+#[derive(Debug)]
+pub struct NotError;
+
+#[derive(Error, Debug)]
+#[error("...")]
+pub enum ErrorEnum {
+    Broken(#[source] NotError),
+}
+
+fn main() {}