Declare std::array members with attribute const [PR101831].

Declare std::array members with attribute const [PR101831].

Resolves:
PR libstdc++/101831 - Spurious maybe-uninitialized warning on std::array::size

libstdc++-v3/ChangeLog:

	PR libstdc++/101831
	* include/std/array (begin): Declare const member function attribute
	const.
	(end, rbegin, rend, size, max_size, empty, data): Same.
	* testsuite/23_containers/array/capacity/empty.cc: Add test cases.
	* testsuite/23_containers/array/capacity/max_size.cc: Same.
	* testsuite/23_containers/array/capacity/size.cc: Same.
	* testsuite/23_containers/array/iterators/begin_end.cc: New test.