Add benchmarks for sorting and heap functions.

Summary:
Benchmarks for std::sort, std::stable_sort, std::make_heap,
std::sort_heap, std::pop_heap and std::push_heap.

The benchmarks are run with integers and strings, and with different
sorted input.

Reviewers: EricWF

Subscribers: christof, mgrang, ldionne, libcxx-commits

Differential Revision: https://reviews.llvm.org/D53978

llvm-svn: 347329