Auto merge of #111717 - Urgau:uplift_fn_null_check, r=oli-obk

Uplift `clippy::fn_null_check` lint

This PR aims at uplifting the `clippy::fn_null_check` lint into rustc.

## `incorrect_fn_null_checks`

(warn-by-default)

The `incorrect_fn_null_checks` lint checks for expression that checks if a function pointer is null.

### Example

```rust
let fn_ptr: fn() = /* somehow obtained nullable function pointer */

if (fn_ptr as *const ()).is_null() { /* ... */ }
```

### Explanation

Function pointers are assumed to be non-null, checking for their nullity is incorrect.

-----

Mostly followed the instructions for uplifting a clippy lint described here: https://github.com/rust-lang/rust/pull/99696#pullrequestreview-1134072751

`@rustbot` label: +I-lang-nominated
r? compiler

compiler/rustc_span/src/symbol.rs

@@ -1152,8 +1152,10 @@ symbols! {
         profiler_runtime,
         ptr,
         ptr_cast_mut,
+        ptr_const_is_null,
         ptr_from_ref,
         ptr_guaranteed_cmp,
+        ptr_is_null,
         ptr_mask,
         ptr_null,
         ptr_null_mut,