Rollup merge of rust-lang#141698 - oli-obk:ctfe-err-flip, r=RalfJung

Use the informative error as the main const eval error message

r? `@RalfJung`

I only did the minimal changes necessary to the const eval error machinery. I'd prefer not to mix test changes with refactorings 😆

Author: matthiaskrgr

tests/ui/indexing_slicing_index.rs

@@ -68,7 +68,7 @@ fn main() {
     // This should be linted, since `suppress-restriction-lint-in-const` default is false.
     const { &ARR[idx4()] };
     //~^ ERROR: indexing may panic
-    //~| ERROR: evaluation of `main
+    //~| ERROR: index out of bounds
 
     let y = &x;
     // Ok, referencing shouldn't affect this lint. See the issue 6021

tests/ui/indexing_slicing_index.stderr

@@ -9,11 +9,11 @@ LL | const REF: &i32 = &ARR[idx()]; // This should be linted, since `suppress-re
    = note: `-D clippy::indexing-slicing` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(clippy::indexing_slicing)]`
 
-error[E0080]: evaluation of `main::{constant#3}` failed
+error[E0080]: index out of bounds: the length is 2 but the index is 4
   --> tests/ui/indexing_slicing_index.rs:69:14
    |
 LL |     const { &ARR[idx4()] };
-   |              ^^^^^^^^^^^ index out of bounds: the length is 2 but the index is 4
+   |              ^^^^^^^^^^^ evaluation of `main::{constant#3}` failed here
 
 note: erroneous constant encountered
   --> tests/ui/indexing_slicing_index.rs:69:5