[libc] Add support for C++20 'atomic_ref' type (#132302)
Summary:
C++20 introduced an atomic reference type, which more easily wraps
around the standard way of dealing with atomics. Instead of a dedicated
type, it allows you to treat an existing allocation as atomic.
This has no users yet, but I'm hoping to use it when I start finalizing
my GPU allocation interface, as it will need to handle atomic values
in-place that can't be done with placement new. Hopefully this is small
enough that we can just keep it in-tree until it's needed, but I'll
accept holding it here until it has a user.
I added one extension to allow implicit conversion and CTAD.
