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

tests/ui/source-struct-unnamed-field-not-error.stderr

@@ -0,0 +1,21 @@
+error[E0599]: the method `as_dyn_error` exists for struct `NotError`, but its trait bounds were not satisfied
+ --> tests/ui/source-struct-unnamed-field-not-error.rs:8:26
+  |
+4 | struct NotError;
+  | ---------------
+  | |
+  | method `as_dyn_error` not found for this struct
+  | doesn't satisfy `NotError: AsDynError<'_>`
+  | doesn't satisfy `NotError: std::error::Error`
+...
+8 | pub struct ErrorStruct(#[source] NotError);
+  |                          ^^^^^^ method cannot be called on `NotError` due to unsatisfied trait bounds
+  |
+  = note: the following trait bounds were not satisfied:
+          `NotError: std::error::Error`
+          which is required by `NotError: AsDynError<'_>`
+note: the trait `std::error::Error` must be implemented
+ --> $RUST/core/src/error.rs
+  |
+  | pub trait Error: Debug + Display {
+  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^