libstdc++: Simplify std::array accessors [PR104719]

libstdc++: Simplify std::array accessors [PR104719]

This removes the __array_traits::_S_ref and __array_traits::_S_ptr
accessors, which only exist to make the special case of std::array<T, 0>
syntactically well-formed.

By changing the empty type used as the std::array<T, 0>::_M_elems data
member to support operator[] and conversion to a pointer, we can write
code using the natural syntax. The indirection through _S_ref and
_S_ptr is removed for the common case, and a function call is only used
for the special case of zero-size arrays.

The invalid member access for zero-sized arrays is changed to use
__builtin_trap() instead of a null dereference. This guarantees a
runtime error if it ever gets called, instead of undefined behaviour
that is likely to get optimized out as unreachable.

libstdc++-v3/ChangeLog:

	PR libstdc++/104719
	* include/std/array (__array_traits::_S_ref): Remove.
	(__array_traits::_S_ptr): Remove.
	(__array_traits<T, 0>::_Type): Define operator[] and operator T*
	to provide an array-like API.
	(array::_AT_Type): Remove public typeef.
	(array::operator[], array::at, array::front, array::back): Use
	index operator to access _M_elems instead of _S_ref.
	(array::data): Use implicit conversion from _M_elems to pointer.
	(swap(array&, array&)): Use __enable_if_t helper.
	(get<I>): Use index operator to access _M_elems.
	* testsuite/23_containers/array/tuple_interface/get_neg.cc:
	Adjust dg-error line numbers.