Non-portable test for SFINAE-friendly std::to_address

https://github.com/llvm/llvm-project/blob/47de55f28438a4a3c3c96157fba66694587d8040/libcxx/test/std/utilities/memory/pointer.conversion/to_address_without_pointer_traits.pass.cpp#L56

This test was added by #65177 which claims to be implementing [LWG3545](https://wg21.link/LWG3545), but that issue only said that `pointer_traits` should be SFINAE-friendly. It didn't require that `to_address` should be SFINAE-friendly.

There is a separate (and unresolved) issue about making `to_address` SFINAE-friendly: [LWG4058](https://wg21.link/lwg4058).

Until LWG4058 is resolved, this test is not portable and relies on a non-standard libc++ extension.

Start with libcxx/test/std/utilities/memory/pointer.conversion/to_address_without_pointer_traits.pass.cpp at line 56, then read the linked discussion of LWG3545 and unresolved LWG4058. Check which behavior the test requires and ensure the test only covers behavior required by the standard rather than a libc++ extension.

Written by the indexing model from the issue text.