[libc++][PSTL] Simplify the partitioning algorithm until we have better data...

[libc++][PSTL] Simplify the partitioning algorithm until we have better data to know how to chunk better

The current chunking strategy is very bad for sorting, and we don't really know how to chunk in general. This fixes the performance problem for sorting.

Reviewed By: ldionne, #libc

Spies: libcxx-commits, krytarowski

Differential Revision: https://reviews.llvm.org/D155531