[libc++] Consider using std::find_if in std::copy_if to figure out where to start copying from

[ldionne]

If std::find_if is fast, this can be beneficial.

We can also potentially find the end of the range to copy using another call to std::find_if. This is probably not worth it unless we have some knowledge of the predicate and iterator cost, but if we did that on random access iterators, we could call memcpy between the two found positions.