Rollup merge of #89897 - jkugelman:must-use-core, r=joshtriplett

Add #[must_use] to remaining core functions

I've run out of compelling reasons to group functions together across crates so I'm just going to go module-by-module. This is everything remaining from the `core` crate.

Ignored by clippy for reasons unknown:

```rust
core::alloc::Layout   unsafe fn for_value_raw<T: ?Sized>(t: *const T) -> Self;
core::any             const fn type_name_of_val<T: ?Sized>(_val: &T) -> &'static str;
```

Ignored by clippy because of `mut`:

```rust
str   fn split_at_mut(&mut self, mid: usize) -> (&mut str, &mut str);
```

<del>
Ignored by clippy presumably because a caller might want `f` called for side effects. That seems like a bad usage of `map` to me.

```rust
core::cell::Ref<'b, T>   fn map<U: ?Sized, F>(orig: Ref<'b, T>, f: F) -> Ref<'b, T>;
core::cell::Ref<'b, T>   fn map_split<U: ?Sized, V: ?Sized, F>(orig: Ref<'b, T>, f: F) -> (Ref<'b, U>, Ref<'b, V>);
```
</del>

Parent issue: #89692

r? ```@joshtriplett```

library/core/src/num/error.rs

@@ -115,6 +115,7 @@ pub enum IntErrorKind {
 
 impl ParseIntError {
     /// Outputs the detailed cause of parsing an integer failing.
+    #[must_use]
     #[stable(feature = "int_error_matching", since = "1.55.0")]
     pub fn kind(&self) -> &IntErrorKind {
         &self.kind