Add benchmarks for std::function.

Summary:
Benchmarks for construct, copy, move, swap, destroy and invoke, with 8
different input states.
For the cases that matter, it tests with and without allowing constant
value propagation from construction into the operation tested.

This also adds helper functions to generate the cartesian product of
different configurations and generate benchmarks for all of them.

Reviewers: EricWF

Subscribers: christof, ldionne, libcxx-commits

Differential Revision: https://reviews.llvm.org/D53087

llvm-svn: 344415