Revert "[libc++] Require C++20 to build the benchmarks."

There are build bots without C++20 support building the benchmarks.

This reverts commit 34acc91642440b8e4bad17acfdbb1314c8f2043e.

GitOrigin-RevId: 171956aab32efa2d2f4999335a3acec15a9472d9
2 files changed