[libcxx/variant] Add a few benchmarks for `std::visit`.

This patch adds a few `std::visit` benchmarks as a starting point.

Reviewed By: ldionne, #libc

Differential Revision: https://reviews.llvm.org/D85419

GitOrigin-RevId: c6f51377e12b343ca2f712b6af6908555f07ad04
4 files changed