libstdc++: Micro-optimisations for lexicographical_compare_three_way

As noted in LWG 3410 the specification in the C++20 draft performs more
iterator comparisons than necessary when the end of either range is
reached. Our implementation followed that specification. This removes
the redundant comparisons so that we do no unnecessary work as soon as
we find that we've reached the end of either range.

The odd-looking return statement is because it generates better code
than the original version that copied the global constants.

	* include/bits/stl_algobase.h (lexicographical_compare_three_way):
	Avoid redundant iterator comparisons (LWG 3410).

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