[libc++] Fix LWG3422 "Issues of seed_seq's constructors"

https://cplusplus.github.io/LWG/issue3422

Also add a static_assert to check the "Mandates:" on the
iterator-pair constructor. Oddly, the `InputIterator` parameter
itself is merely preconditioned, not constrained, to satisfy the
input iterator requirements.

Also drive-by rename `init` to `__init`.

Differential Revision: https://reviews.llvm.org/D117962