This fixes the issue that prevents `std::string::size_type` from being imported into Swift: `size_type` is imported before its parent struct, and we're skipping it when importing the struct afterwards.
This is caused by an out-of-line decl for `std::string::npos`:
```cpp
template<class _CharT, class _Traits, class _Allocator>
_LIBCPP_FUNC_VIS
const typename basic_string<_CharT, _Traits, _Allocator>::size_type
basic_string<_CharT, _Traits, _Allocator>::npos;
```
When importing `npos`, we first import `size_type`, which triggers the issue.
