test: add regression test for issue #3319

Add test verifying structs with unknown attributes (CXCursor_UnexposedAttr)
do not generate layout tests that could fail at compile time on some platforms.

Author: naoNao89

bindgen-tests/tests/expectations/tests/issue-3319-unknown-attr.rs

@@ -0,0 +1,14 @@
+// This test verifies that structs with unknown attributes (CXCursor_UnexposedAttr)
+// do not generate incorrect layout assertions that would fail at compile time.
+// 
+// The found_unknown_attr flag is set when bindgen encounters attributes it cannot
+// fully interpret, which may affect layout in ways bindgen cannot predict.
+// 
+// See: https://github.com/rust-lang/rust-bindgen/issues/3319
+
+// __ms_struct__ is an attribute that triggers CXCursor_UnexposedAttr
+// and sets found_unknown_attr = true
+struct __attribute__((__ms_struct__)) StructWithUnknownAttr {
+    int x;
+    double y;
+};