move stage2 compile errors out of special folder

test/cases/compile_errors/union_duplicate_enum_field.zig

@@ -0,0 +1,17 @@
+const E = enum { a, b };
+const U = union(E) {
+    a: u32,
+    a: u32,
+};
+
+export fn foo() void {
+    var u: U = .{ .a = 123 };
+    _ = u;
+}
+
+// error
+// target=native
+//
+// :4:5: error: duplicate union field: 'a'
+// :3:5: note: other field here
+// :2:11: note: union declared here