refactor: Move Apple OSVersion (back) to rustc_target

Also convert OSVersion into a proper struct for better type-safety.

compiler/rustc_target/src/lib.rs

@@ -12,6 +12,7 @@
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
 #![doc(rust_logo)]
 #![feature(assert_matches)]
+#![feature(debug_closure_helpers)]
 #![feature(iter_intersperse)]
 #![feature(let_chains)]
 #![feature(rustc_attrs)]