Highlight the const function if error happened because of a bound on the impl block

Currently, for the following code, the compiler produces the errors like the
following error:
```rust
struct Type<T>

impl<T: Clone> Type<T> {
	fn const f() {}
}
```
```text
error[E0658]: trait bounds other than `Sized` on const fn parameters are unstable
 --> ./test.rs:3:6
  |
3 | impl<T: Clone> Type<T> {
  |      ^
  |
  = note: see issue #57563 <https://github.com/rust-lang/rust/issues/57563> for more information
  = help: add `#![feature(const_fn_trait_bound)]` to the crate attributes to enable
```

This can be confusing (especially to newcomers) since the error mentions
"const fn parameters", but highlights only the impl.

This commits adds function highlighting, changing the error to the following:

```text
error[E0658]: trait bounds other than `Sized` on const fn parameters are unstable
 --> ./test.rs:3:6
  |
3 | impl<T: Clone> Type<T> {
  |      ^
4 |     pub const fn f() {}
  |     ---------------- function declared as const here
  |
  = note: see issue #57563 <https://github.com/rust-lang/rust/issues/57563> for more information
  = help: add `#![feature(const_fn_trait_bound)]` to the crate attributes to enable
```

compiler/rustc_const_eval/src/transform/check_consts/ops.rs

@@ -599,12 +599,21 @@ pub mod ty {
         }
 
         fn build_error(&self, ccx: &ConstCx<'_, 'tcx>, span: Span) -> DiagnosticBuilder<'tcx> {
-            feature_err(
+            let mut builder = feature_err(
                 &ccx.tcx.sess.parse_sess,
                 sym::const_fn_trait_bound,
                 span,
                 "trait bounds other than `Sized` on const fn parameters are unstable",
-            )
+            );
+
+            match ccx.fn_sig() {
+                Some(fn_sig) if !fn_sig.span.contains(span) => {
+                    builder.span_label(fn_sig.span, "function declared as const here");
+                }
+                _ => {}
+            }
+
+            builder
         }
     }