libstdc++: Refactor seed sequence constraints in <random>

libstdc++: Refactor seed sequence constraints in <random>

Every use of _If_seed_seq in <random> and <ext/random> uses it with
enable_if. We can just move the enable_if into the helper alias instead
of repeating it everywhere.

libstdc++-v3/ChangeLog:

	* include/bits/random.h (__is_seed_seq): Replace with ...
	(_If_seed_seq_for): ... this.
	* include/ext/random: Adjust to use _If_seed_seq_for.