libstdc++: Make std::expected trivially copy/move assignable (LWG 4026)HEADtrunkmaster

This is the subject of two NB comments on C++26 which seem likely to be
approved. We're allowed to make this change as QoI anyway, even if it
isn't approved for the standard, and it should apply to C++23 as well to
avoid ABI changes between C++23 and C++26.

As shown in the updates to the test, defaulted special members can have
noexcept(false) even if they would be noexcept(true) by default. The new
defaulted operator= overloads added by this commit have conditional
noexcept-specifiers that match the conditions of the non-trivial
assignments, propagating any noexcept(false) on trivial special members
of the T and E types. We could strengthen the noexcept for the trivial
operators, but propagating the conditions from the underlying types is
probably what users expect, if they've bothered to put noexcept(false)
on their defaulted special members.

libstdc++-v3/ChangeLog:

	* include/std/expected (__expected::__trivially_replaceable)
	(__expected::__usable_for_assign)
	(__expected::__usable_for_trivial_assign)
	(__expected::__can_reassign_type): New concepts.
	(expected::operator=): Adjust constraints
	on existing overloads and add defaulted overload.
	(expected<cv void, E>::operator=): Likewise.
	* testsuite/20_util/expected/requirements.cc: Check for trivial
	and nothrow properties of assignments.

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