Auto merge of #84217 - crlf0710:remove_main_attr_pure, r=petrochenkov

Remove #[main] attribute.

This removes the #[main] attribute support from the compiler according to the decisions within #29634. For existing use cases within test harness generation, replaced it with a newly-introduced internal attribute `#[rustc_main]`.

This is first part extracted from #84062 .

Closes #29634.

r? `@petrochenkov`

src/tools/clippy/tests/ui/crate_level_checks/entrypoint_recursion.rs

@@ -1,11 +1,11 @@
 // ignore-macos
 // ignore-windows
 
-#![feature(main)]
+#![feature(rustc_attrs)]
 
 #[warn(clippy::main_recursion)]
 #[allow(unconditional_recursion)]
-#[main]
+#[rustc_main]
 fn a() {
     println!("Hello, World!");
     a();