P3981R2: Better return types in `std::inplace_vector` and `std::exception_ptr_cast`

**Link:** https://wg21.link/P3981R2

Feature-test macro (updated):
```c++
#define __cpp_lib_inplace_vector 202603L // also in
#define __cpp_lib_exception_ptr_cast 202603L // also in
```

Note that [P4022R0](https://wg21.link/P4022R0) also updates `__cpp_lib_inplace_vector`.