Auto merge of #83948 - ABouttefeux:lint-nullprt-deref, r=RalfJung

add lint deref_nullptr detecting when a null ptr is dereferenced

fixes #83856
changelog: add lint that detect code like
```rust
unsafe {
      &*core::ptr::null::<i32>()
 };
unsafe {
     addr_of!(std::ptr::null::<i32>())
};
let x: i32 = unsafe {*core::ptr::null()};
let x: i32 = unsafe {*core::ptr::null_mut()};
unsafe {*(0 as *const i32)};
unsafe {*(core::ptr::null() as *const i32)};
```
```
warning: Dereferencing a null pointer causes undefined behavior
 --> src\main.rs:5:26
  |
5 |     let x: i32 = unsafe {*core::ptr::null()};
  |                          ^^^^^^^^^^^^^^^^^^
  |                          |
  |                          a null pointer is dereferenced
  |                          this code causes undefined behavior when executed
  |
  = note: `#[warn(deref_nullptr)]` on by default
```

Limitation:
It does not detect code like
```rust
const ZERO: usize = 0;
unsafe {*(ZERO as *const i32)};
```
or code where `0` is not directly a literal

compiler/rustc_span/src/symbol.rs

@@ -900,6 +900,8 @@ symbols! {
         profiler_runtime,
         ptr_guaranteed_eq,
         ptr_guaranteed_ne,
+        ptr_null,
+        ptr_null_mut,
         ptr_offset_from,
         pub_macro_rules,
         pub_restricted,