[libc++] Trim down associative container benchmark sizes (#212252)

Based on local measurements, the associative containers are by far the
slowest benchmarks to run. While there is value in benchmarking small
and large sizes, we can significantly cut down benchmark times by
stripping down to only 2 sizes (from 4).

With this patch, the benchmarks go from 3124s to 1766s across the
associative container benchmarks, which is -43% of their execution time,
and corresponds to ~15% of the entire benchmark suite.

Towards #173032

GitOrigin-RevId: c2ae9308090e26cc6e3e4ab64c06326061c9572c