PR libstdc++/80137 use std::nextafter instead of looping

	PR libstdc++/80137
	* include/bits/random.tcc (generate_canonical): Use std::nextafter
	or numeric_limits::epsilon() to reduce out-of-range values.
	* testsuite/26_numerics/random/uniform_real_distribution/operators/
	64351.cc: Verify complexity requirement is met.

From-SVN: r246542