Auto merge of #18252 - ShoyuVanilla:issue-15799, r=Veykril

fix: Do not consider mutable usage of deref to `*mut T` as deref_mut

Fixes #15799

We are doing some heuristics for deciding whether the given deref is deref or deref_mut here;

https://github.com/rust-lang/rust-analyzer/blob/5982d9c420d0dc90739171829f0d2e9c80d98979/crates/hir-ty/src/infer/mutability.rs#L182-L200

But this heuristic is erroneous if we are dereferencing to a mut ptr and normally those cases are filtered out here as builtin;

https://github.com/rust-lang/rust-analyzer/blob/5982d9c420d0dc90739171829f0d2e9c80d98979/crates/hir-ty/src/mir/lower/as_place.rs#L165-L177

Howerver, this works not so well if the given dereferencing is double dereferencings like the case in the #15799.

```rust
struct WrapPtr(*mut u32);

impl core::ops::Deref for WrapPtr {
    type Target = *mut u32;
    fn deref(&self) -> &Self::Target {
        &self.0
    }
}

fn main() {
    let mut x = 0u32;
    let wrap = WrapPtr(&mut x);
    unsafe {
        **wrap = 6;
    }
}
```

Here are two - outer and inner - dereferences here, and the outer dereference is marked as deref_mut because there is an assignment operation.
And this deref_mut marking is propagated into the inner dereferencing.
In the later MIR lowering, the outer dereference is filtered out as it's expr type is `*mut u32`, but the expr type in the inner dereference is an ADT, so this false-mutablility is not filtered out.

This PR cuts propagation of this false mutablilty chain if the expr type is mut ptr.
Since this happens before the resolve_all, it may have some limitations when the expr type is determined as mut ptr at the very end of inferencing, but I couldn't find simple fix for it 🤔

Author: bors