Rollup merge of #88775 - pnkfelix:revert-anon-union-parsing, r=davidtwco

Revert anon union parsing

Revert PR #84571 and #85515, which implemented anonymous union parsing in a manner that broke the context-sensitivity for the `union` keyword and thus broke stable Rust code.

Fix #88583.

src/test/ui/parser/issue-88583-union-as-ident.rs

@@ -0,0 +1,15 @@
+// check-pass
+
+#![allow(non_camel_case_types)]
+
+struct union;
+
+impl union {
+    pub fn new() -> Self {
+        union { }
+    }
+}
+
+fn main() {
+    let _u = union::new();
+}