libstdc++: Fixes for std::expected

libstdc++: Fixes for std::expected

This fixes some bugs in the swap functions for std::expected.

It also disables the noexcept-specifiers for equality operators, because
those are problematic when querying whether a std::expected is equality
comparable. The operator==(const expected<T,E>&, const U&) function is
not constrained, so is viable for comparing expected<T,E> with
expected<void,G>, but then we get an error from the noexcept-specifier.

libstdc++-v3/ChangeLog:

	* include/std/expected (expected::_M_swap_val_unex): Guard the
	correct object.
	(expected::swap): Move is_swappable
	requirement from static_assert to constraint.
	(swap): Likewise.
	(operator==): Remove noexcept-specifier.
	* testsuite/20_util/expected/swap.cc: Check swapping of
	types without non-throwing move constructor. Check constraints
	on swap.
	* testsuite/20_util/expected/unexpected.cc: Check constraints on
	swap.
	* testsuite/20_util/expected/equality.cc: New test.