Possible issues with deduction guides from p1518r2

Does https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2021/p1518r2.html somehow also apply to indirect?

I.e. should it be:

```diff
- constexpr indirect(allocator_arg_t, const Allocator& a, const indirect& other);
+ constexpr indirect(allocator_arg_t, const type_identity_t& a, const indirect& other);

- constexpr indirect(allocator_arg_t, const Allocator& a, indirect&& other);
+ constexpr indirect(allocator_arg_t, const type_identity_t& a, indirect&& other);
```

I am not sure, but I got reminded of P1518 and at least wanted to mention it.

Possibly related: https://github.com/microsoft/STL/issues/4060

We're currently consistent with tuple: https://eel.is/c++draft/tuple.tuple which might be the right way to be.

Taken from conversation with @Ukilele on another issue #497