libstdc++: Make relational operators work with const guarded iterators (PR 92472)

libstdc++: Make relational operators work with const guarded iterators (PR 92472)

This is a correct fix for the incorrect cppcheck suggestion to make
these parameters const. In order to that, the dereference operators need
to be const. The conversions to the underlying iterator can be const
too.

	PR c/92472
	* include/parallel/multiway_merge.h (_GuardedIterator::operator*)
	(_GuardedIterator::operator _RAIter, _UnguardedIterator::operator*)
	(_UnguardedIterator::operator _RAIter): Add const qualifier.
	(operator<(_GuardedIterator&, _GuardedIterator&)
	(operator<=(_GuardedIterator&, _GuardedIterator&)
	(operator<(_UnguardedIterator&, _UnguardedIterator&)
	(operator<=(_UnguardedIterator&, _UnguardedIterator&): Change
	parameters to const references.