compiler: Use size_of from the prelude instead of imported

Use `std::mem::{size_of, size_of_val, align_of, align_of_val}` from the
prelude instead of importing or qualifying them.

These functions were added to all preludes in Rust 1.80.

compiler/rustc_trait_selection/src/traits/const_evaluatable.rs

@@ -2,7 +2,7 @@
 //!
 //! For concrete constants, this is fairly simple as we can just try and evaluate it.
 //!
-//! When dealing with polymorphic constants, for example `std::mem::size_of::<T>() - 1`,
+//! When dealing with polymorphic constants, for example `size_of::<T>() - 1`,
 //! this is not as easy.
 //!
 //! In this case we try to build an abstract representation of this constant using