stage2: bitsize of packed struct should trigger backing int ty check

Closes #13398

test/cases/compile_errors/bitsize_of_packed_struct_checks_backing_int_ty.zig

@@ -0,0 +1,13 @@
+const Foo = packed struct(u32) {
+    x: u1,
+};
+fn bar(_: Foo) callconv(.C) void {}
+pub export fn entry() void {
+    bar(.{ .x = 0 });
+}
+
+// error
+// backend=stage2
+// target=native
+//
+// :1:27: error: backing integer type 'u32' has bit size 32 but the struct fields have a total bit size of 1