Use std::piecewise_constant_distribution instead of ad-hoc binary search.

Summary:
Fix the issue with the most recently discovered unit receiving much less attention.

Note: this is the second attempt (prev: r258473). Now, libc++ build is fixed.

Reviewers: aizatsky, kcc

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D16487

llvm-svn: 258571