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};
    }
}
```

Author: philberty

gcc/rust/typecheck/rust-hir-const-fold.h

@@ -427,6 +427,15 @@ class ConstFoldExpr : public ConstFoldBase
       = ctx->get_backend ()->negation_expression (op, negated_expr, location);
   }
 
+  void visit (HIR::ArrayIndexExpr &expr) override
+  {
+    Bexpression *array = ConstFoldExpr::fold (expr.get_array_expr ());
+    Bexpression *index = ConstFoldExpr::fold (expr.get_index_expr ());
+
+    folded = ctx->get_backend ()->array_index_expression (array, index,
+							  expr.get_locus ());
+  }
+
 private:
   ConstFoldExpr ()
     : ConstFoldBase (), folded (ctx->get_backend ()->error_expression ())

gcc/testsuite/rust/compile/torture/constant3.rs

@@ -0,0 +1,10 @@
+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;
+}