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_errors/src/diagnostic.rs

@@ -490,7 +490,7 @@ pub struct Diag<'a, G: EmissionGuarantee = ErrorGuaranteed> {
 // would be bad.
 impl<G> !Clone for Diag<'_, G> {}
 
-rustc_data_structures::static_assert_size!(Diag<'_, ()>, 3 * std::mem::size_of::<usize>());
+rustc_data_structures::static_assert_size!(Diag<'_, ()>, 3 * size_of::<usize>());
 
 impl<G: EmissionGuarantee> Deref for Diag<'_, G> {
     type Target = DiagInner;