In `const fn` in a macro call, raw ptr deref does not work even inside `unsafe`

The following code should build, but for some reason it does not:
```rust
#![feature(const_raw_ptr_deref)]
#![feature(raw_ref_macros)]

use std::ptr;

const fn test_fn(x: *const i32) {
    let x2 = unsafe { ptr::raw_const!(*x) };
}
```
Removing the `const` or adding a `const_fn` feature gate makes it build.

Cc @oli-obk @petrochenkov 