libstdc++ Two simplifications for lexicographical_compare

libstdc++ Two simplifications for lexicographical_compare

	* include/bits/ranges_algo.h (__lexicographical_compare_fn): Declare
	variables in smaller scope and avoid calling ranges::distance when we
	know they are pointers. Remove statically-unreachable use of
	__builtin_unreachable().
	* include/bits/stl_algobase.h (__lexicographical_compare::__lc):
	Define inline.