[libcxx] Speeding up partition_point/lower_bound/upper_bound

This is a re-application of r345525, which had been reverted by fear of
a regression.

Reviewed as https://reviews.llvm.org/D53994.
Thanks to Denis Yaroshevskiy for the patch.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@349358 91177308-0d34-0410-b5e6-96231b3b80d8