LWG-3595 Exposition-only classes proxy and postfix-proxy for `common_iterator` should be fully `constexpr`

[AlexGuteniev]

LWG-3595 asks for constexpr members of common_iterator proxies proxy and postfix-proxy.

proxy is returned by operator-> so it is observable, need to implement the change for it.
postfix-proxy is not returned and not observable, in fact the STL does not have it. isadded by #1991 , and constexpr is observable.

Also noexcept for indirection operator is no longer strengthened.

Need to change the _Proxy_base, _Arrow_proxy and _Postfix_proxy classes:

STL/stl/inc/iterator

Lines 818 to 824 in 8833270

	struct _Proxy_base {
	iter_value_t<_Iter> _Keep;
	explicit _Proxy_base(iter_reference_t<_Iter>&& _Right) noexcept(
	is_nothrow_constructible_v<iter_value_t<_Iter>, iter_reference_t<_Iter>>) // strengthened
	: _Keep(_STD forward<iter_reference_t<_Iter>>(_Right)) {}
	};

STL/stl/inc/iterator

Lines 890 to 899 in 8833270

	class _Arrow_proxy : private _Proxy_base {
	public:
	friend common_iterator;
	using _Proxy_base::_Proxy_base;
	_NODISCARD const iter_value_t<_Iter>* operator->() const noexcept /* strengthened */ {
	return _STD addressof(this->_Keep);
	}
	};

STL/stl/inc/iterator

Lines 924 to 933 in 8833270

	class _Postfix_proxy : private _Proxy_base {
	public:
	friend common_iterator;
	using _Proxy_base::_Proxy_base;
	_NODISCARD const iter_value_t<_Iter>& operator*() const noexcept /* strengthened */ {
	return this->_Keep;
	}
	};

[cpplearner]

#1991 will add the postfix-proxy class. If someone wants to work on LWG-3595, they probably need to wait until #1991 is merged.