Add constant folding to ArrayIndexExpr

This ensures that constexpr is enforced for array-index-exprs resulting in
gimple like this:

```rust
fn main() {
    const A: [i32; 3] = [1, 2, 3];
    const B: i32 = A[1];
    const C: usize = 42;
    const D: i32 = 7;

    let _a = C;
    let _b: [i32; C] = [0; C];
    let _c = B + D;
}
```

```c
void main ()
{
  const usize _a;
  i32 _b[42];
  const i32 _c;

  try
    {
      _a = 42;
      _b = {};
      _1 = 2;
      _c = _1 + 7;
    }
  finally
    {
      _b = {CLOBBER};
    }
}
```

0 files changed, 0 insertions, 0 deletions