Rollup merge of #76075 - marmeladema:remove-once-cell-crate, r=matklad

datastructures: replace `once_cell` crate with an impl from std

Fixes #75700

r? @matklad

We might need a perf run for this change.

compiler/rustc_middle/src/lib.rs

@@ -38,6 +38,7 @@
 #![feature(exhaustive_patterns)]
 #![feature(extern_types)]
 #![feature(nll)]
+#![feature(once_cell)]
 #![feature(option_expect_none)]
 #![feature(or_patterns)]
 #![feature(min_specialization)]