Auto merge of #25570 - oli-obk:const_indexing, r=nikomatsakis

This PR allows the constant evaluation of index operations on constant arrays and repeat expressions. This allows index expressions to appear in the expression path of the length expression of a repeat expression or an array type.

An example is

```rust
const ARR: [usize; 5] = [1, 2, 3, 4, 5];
const ARR2: [usize; ARR[1]] = [42, 99];
```

In most other locations llvm's const evaluator figures it out already. This is not specific to index expressions and could be remedied in the future.

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