Fixed build on systems needing explicit <utility> in RegexPattern.cc.

Fixed build on systems needing explicit <utility> in RegexPattern.cc.

... after r14795.

Also removed bogus comments implying that std::move() does something to
its argument. That function is just a type cast; it does nothing else!
We use std::move() so that the compiler picks a move constructor or
assignment (where available). In case of RegexPattern::flags, using
std::move() has no effect because integers do not have those move
methods. However, it may be a good idea to use std::move() anyway
because the type of RegexPattern::flags might change in the future and
for consistency sake. Thus, I did not remove std::move(), only comments.