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

Some work-in-progress patches for the format header contain benchmarks.
The format header requires C++20 to build. This is a preparation to make
it easy to add these benchmarks.

Reviewed By: ldionne, #libc

Differential Revision: https://reviews.llvm.org/D96057

GitOrigin-RevId: 34acc91642440b8e4bad17acfdbb1314c8f2043e
2 files changed