don't try to blame tuple fields for immutability

Tuple fields don't have an `&T` in their declaration that can be changed
to `&mut T` - skip them..

Fixes #41104.

src/test/ui/did_you_mean/issue-39544.rs

@@ -51,3 +51,9 @@ pub fn with_arg(z: Z, w: &Z) {
     let _ = &mut z.x;
     let _ = &mut w.x;
 }
+
+pub fn with_tuple() {
+    let mut y = 0;
+    let x = (&y,);
+    *x.0 = 1;
+}

src/test/ui/did_you_mean/issue-39544.stderr

@@ -90,5 +90,11 @@ error: cannot borrow immutable field `w.x` as mutable
 52 |     let _ = &mut w.x;
    |                  ^^^ cannot mutably borrow immutable field
 
-error: aborting due to 11 previous errors
+error: cannot assign to immutable borrowed content `*x.0`
+  --> $DIR/issue-39544.rs:58:5
+   |
+58 |     *x.0 = 1;
+   |     ^^^^^^^^ cannot borrow as mutable
+
+error: aborting due to 12 previous errors