Update test with improved diagnostics

Now, `derived_from_illegal_borrow` is also applied to reborrows, so we
don't show the user a useless error message.

Author: ecstatic-morse

src/test/ui/consts/const-multi-ref.rs

@@ -1,8 +1,11 @@
+// Ensure that we point the user to the erroneous borrow but not to any subsequent borrows of that
+// initial one.
+
 const _X: i32 = {
     let mut a = 5;
-    let p = &mut a;      //~ ERROR references in constants may only refer to immutable values
+    let p = &mut a; //~ ERROR references in constants may only refer to immutable values
 
-    let reborrow = {p};  //~ ERROR references in constants may only refer to immutable values
+    let reborrow = {p};
     let pp = &reborrow;
     let ppp = &pp;
     ***ppp

src/test/ui/consts/const-multi-ref.stderr

@@ -1,15 +1,9 @@
 error[E0017]: references in constants may only refer to immutable values
-  --> $DIR/const-multi-ref.rs:3:13
+  --> $DIR/const-multi-ref.rs:6:13
    |
 LL |     let p = &mut a;
    |             ^^^^^^ constants require immutable values
 
-error[E0017]: references in constants may only refer to immutable values
-  --> $DIR/const-multi-ref.rs:5:21
-   |
-LL |     let reborrow = {p};
-   |                     ^ constants require immutable values
-
-error: aborting due to 2 previous errors
+error: aborting due to previous error
 
 For more information about this error, try `rustc --explain E0017`.