[libc++] Implement [P0769] "Add shift to algorithm" (shift_left, shift_right)

I believe this is a complete implementation of std::shift_left and std::shift_right from
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0769r2.pdf

Some test cases copied-with-modification from D60027.

Differential Revision: https://reviews.llvm.org/D93819