libstdc++: Add [[nodiscard]] to std::span members

libstdc++: Add [[nodiscard]] to std::span members

All std::span member functions are pure functions that have no side
effects. They are only useful for their return value, so they should all
warn if that value is not used.

libstdc++-v3/ChangeLog:

	* include/std/span (span, as_bytes, as_writable_bytes): Add
	[[nodiscard]] attribute on all non-void functions.
	* testsuite/23_containers/span/back_assert_neg.cc: Suppress
	nodiscard warning.
	* testsuite/23_containers/span/back_neg.cc: Likewise.
	* testsuite/23_containers/span/first_2_assert_neg.cc: Likewise.
	* testsuite/23_containers/span/first_assert_neg.cc: Likewise.
	* testsuite/23_containers/span/first_neg.cc: Likewise.
	* testsuite/23_containers/span/front_assert_neg.cc: Likewise.
	* testsuite/23_containers/span/front_neg.cc: Likewise.
	* testsuite/23_containers/span/index_op_assert_neg.cc: Likewise.
	* testsuite/23_containers/span/index_op_neg.cc: Likewise.
	* testsuite/23_containers/span/last_2_assert_neg.cc: Likewise.
	* testsuite/23_containers/span/last_assert_neg.cc: Likewise.
	* testsuite/23_containers/span/last_neg.cc: Likewise.
	* testsuite/23_containers/span/subspan_2_assert_neg.cc:
	Likewise.
	* testsuite/23_containers/span/subspan_3_assert_neg.cc:
	Likewise.
	* testsuite/23_containers/span/subspan_4_assert_neg.cc:
	Likewise.
	* testsuite/23_containers/span/subspan_5_assert_neg.cc:
	Likewise.
	* testsuite/23_containers/span/subspan_6_assert_neg.cc:
	Likewise.
	* testsuite/23_containers/span/subspan_assert_neg.cc: Likewise.
	* testsuite/23_containers/span/subspan_neg.cc: Likewise.
	* testsuite/23_containers/span/nodiscard.cc: New test.