add compile error test case for new error

test/cases/compile_errors/packed_union_fields_mismatch.zig

@@ -0,0 +1,14 @@
+export fn entry1() void {
+    _ = packed union {
+        a: u1,
+        b: u2,
+    };
+}
+
+// error
+// backend=stage2
+// target=native
+//
+// :2:16: error: packed union has fields with mismatching bit sizes
+// :3:12: note: 1 bits here
+// :4:12: note: 2 bits here