libstdc++: Add nodiscard attribute to cast-like functions

libstdc++: Add nodiscard attribute to cast-like functions

Add [[nodiscard]] to functions that are effectively just a static_cast,
as per P2351. Also add it to std::addressof.

libstdc++-v3/ChangeLog:

	* include/bits/move.h (forward, move, move_if_noexcept)
	(addressof): Add _GLIBCXX_NODISCARD.
	* include/bits/ranges_cmp.h (identity::operator()): Add
	nodiscard attribute.
	* include/c_global/cstddef (to_integer): Likewise.
	* include/std/bit (bit_cast): Likewise.
	* include/std/utility (as_const, to_underlying): Likewise.