Sema: require reified packed struct fields to have zero alignment

test/cases/compile_errors/packed_struct_field_alignment_unavailable_for_reify_type.zig

@@ -0,0 +1,11 @@
+export fn entry() void {
+    _ = @Type(.{ .Struct = .{ .layout = .Packed, .fields = &.{
+        .{ .name = "one", .field_type = u4, .default_value = null, .is_comptime = false, .alignment = 2 },
+    }, .decls = &.{}, .is_tuple = false } });
+}
+
+// error
+// backend=stage2
+// target=native
+//
+// :2:9: error: alignment in a packed struct field must be set to 0