libstdc++: Fix constraint recursion in std::expected's operator== [PR119714]

This std::expected friend operator== is prone to constraint recursion
after CWG 2369 for the same reason as basic_const_iterator's comparison
operators were before the r15-7757-g4342c50ca84ae5 workaround.  This
patch works around the constraint recursion here in a similar manner,
by making the function parameter of type std::expected dependent in a
trivial way.

	PR libstdc++/119714
	PR libstdc++/112490

libstdc++-v3/ChangeLog:

	* include/std/expected (expected::operator==): Replace
	non-dependent std::expected function parameter with a dependent
	one of type expected<_Vp, _Er> where _Vp matches _Tp.
	* testsuite/20_util/expected/119714.cc: New test.

Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>
Reviewed-by: Jonathan Wakely <jwakely@redhat.com>
(cherry picked from commit 91bc8169edd9038d78f38bd813287d72e6345c26)