[libc++] Allow the use of extensions in the implementation (#79532)

We've talked about allowing extensions on
[discourse](https://discourse.llvm.org/t/rfc-use-language-extensions-from-future-standards-in-libc/71898/5)
and in a libc++ monthly meeting and agreed to test it out in the LLVM 18
release. We've done that with the `tuple` constructor overload set
(using conditional `explicit`). Since we haven't heard about any
breakages, it seems safe to do. This patch enables the use of extension
from later C++ standards inside the versioned `std` namespaces. This
should be good enough, since almost all of our code is inside that
namespace. This approach also avoids the use of extensions inside the
test `std` suite. That part of the code base should stay clean, since
it's a test suite that is also used by other vendors to test their
implementations.

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