As suggested in D117966.
These conditional noexcepts are *permitted* by the Standard (as long
as there were no mistakes in them, I guess); but not *mandated*.
The Standard doesn't put any noexcept-specifications on these member functions.
The same logic would apply to `transform_view::iterator::operator*`
and `transform_view::iterator::operator[]`, but the Standard mandates
conditional noexcept on `iter_move(transform_view::iterator)`, and
I think it doesn't make much sense to say "moving from this iterator
is conditionally noexcept but not-moving from it is noexcept(false),"
so I'm leaving transform_view alone for now.
Differential Revision: https://reviews.llvm.org/D119374
7bdf41653c