[PATCH] libstdc++: add support for cv-qualified types in atomic_ref (P3323R1)

[Giuseppe D'Angelo]

Hello,

The attached patch implements P3323R1, a DR against C++20 (11?), which 
fixes LWG 4069 and 3508 by clarifying that std::atomic_ref<cv T> is 
meant to be supported (whereas std::atomic<cv T> is meant to be 
unsupported).

I've tried to keep the refactorings to a minimum and follow the 
structure of the existing code, where atomic_ref<T> dispatched to a 
__atomic_ref<T> base class which had specializations for various kinds 
of T (integral, fp, pointer). The mutating operations on an 
atomic_ref<T> are now constrained on whether T is const or not, so I 
split __atomic_ref into a further subclass (__atomic_ref_base) that 
contains the non-mutating operations. __atomic_ref inherits from _base 
and adds the mutating operations if T is non const.

Thanks,
-- 
Giuseppe D'Angelo
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