Modifications to the algorithm sort benchmark

Summary:
Modifies the algorithm sort bench:
- shows sorting time per element, instead of sorting time per array.
This would make comparison between different sizes of arrays easier.
- adds std::pair benchmark cases.
- uses a large number of arrays to benchmark, instead of repeatedly sorting the same array.
* sorting the same array again and again would not show actual sorting performance over randomized data sets.

Reviewers: EricWF, #libc, mvels

Reviewed By: EricWF, #libc, mvels

Subscribers: mgrang, libcxx-commits

Tags: #libc

Differential Revision: https://reviews.llvm.org/D81770