libstdc++: Prefer const T to std::add_const_t<T>

For any typedef-name or template parameter, T, add_const_t<T> is
equivalent to T const, so we can avoid instantiating the std::add_const
class template and just say T const (or const T).

This isn't true for a non-typedef like int&, where int& const would be
ill-formed, but we shouldn't be using add_const_t<int&> anyway, because
we know what that type is.

The only place we need to continue using std::add_const is in the
std::bind implementation where it's used as a template template
parameter to be applied as a metafunction elsewhere.

libstdc++-v3/ChangeLog:

	* include/bits/stl_iterator.h (__iter_to_alloc_t): Replace
	add_const_t with const-qualifier.
	* include/bits/utility.h (tuple_element<N, cv T>): Likewise for
	all cv-qualifiers.
	* include/std/type_traits (add_const, add_volatile): Replace
	typedef-declaration with using-declaration.
	(add_cv): Replace add_const and add_volatile with cv-qualifiers.
	* include/std/variant (variant_alternative<N, cv T>): Replace
	add_const_t, add_volatile_t and add_cv_t etc. with cv-qualifiers.

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