libstdc++: Fix iterator caching inside range adaptors [PR100479]

This fixes two issues with our iterator caching as described in detail
in the PR.  Since we recently added the __non_propagating_cache class
template as part of r12-336 for P2328, this patch just rewrites the
problematic _CachedPosition partial specialization in terms of this
class template.

For the offset partial specialization, it's safe to propagate the cached
offset on copy/move, but we should still invalidate the cached offset in
the source object on move.

libstdc++-v3/ChangeLog:

	PR libstdc++/100479
	* include/std/ranges (__detail::__non_propagating_cache): Move
	definition up to before that of _CachedPosition.  Make base
	class _Optional_base protected instead of private.  Add const
	overload for operator*.
	(__detail::_CachedPosition): Rewrite the partial specialization
	for forward ranges as a derived class of __non_propagating_cache.
	Remove the size constraint on the partial specialization for
	random access ranges.  Add copy/move/copy-assignment/move-assignment
	members to the offset partial specialization for random
	access ranges that propagate the cached value but additionally
	invalidate it in the source object on move.
	* testsuite/std/ranges/adaptors/100479.cc: New test.

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