Update fbcode and xplat Rust toolchain

Summary:
Release notes: https://blog.rust-lang.org/2024/06/13/Rust-1.79.0.html

Stabilized features:

- `absolute_path`
- `associated_type_bounds`
- `inline_const`
- `pointer_is_aligned`
- `slice_ptr_len`

This release raises a new warning when a trait impl contains a signature that is more refined than the corresponding signature in the trait, impacting `buck2_query`.

```lang=text
warning: impl trait in impl method signature does not match trait method signature
  --> fbcode/buck2/app/buck2_query/src/query/environment/tests.rs:73:30
   |
73 |     fn deps<'a>(&'a self) -> Box<dyn Iterator<Item = &'a Self::Key> + Send + 'a> {
   |                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
  ::: fbcode/buck2/app/buck2_query/src/query/environment.rs:98:30
   |
98 |     fn deps<'a>(&'a self) -> impl Iterator<Item = &'a Self::Key> + Send + 'a;
   |                              ----------------------------------------------- return type from trait method defined here
   |
   = note: add `#[allow(refining_impl_trait)]` if it is intended for this to be part of the public API of this crate
   = note: we are soliciting feedback, see issue #121718 <rust-lang/rust#121718> for more information
   = note: `#[warn(refining_impl_trait_internal)]` on by default
help: replace the return type so that it matches the trait
   |
73 |     fn deps<'a>(&'a self) -> impl Iterator<Item = &'a <Self as node::LabeledNode>::Key> + std::marker::Send + 'a {
   |                              ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
```

It also contains changes to the `dead_code` lint, which are addressed in {D59622902} and {D59623034}.

Reviewed By: zertosh

Differential Revision: D58982552

fbshipit-source-id: a17eaf91d07234209fddf5cbdebce4424dda57ae

Author: David Tolnay

rust-toolchain

@@ -1,2 +1,2 @@
 [toolchain]
-channel = "nightly-2024-03-17"
+channel = "nightly-2024-04-28"