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: I had to change the seed for one test to make it pass. Alternatively,
the number of runs could be increased. I believe that the average time of
'foo' discovery is not increased, just seed=1 was particularly convenient
for the previous PRNG scheme used.

Reviewers: aizatsky, kcc

Subscribers: llvm-commits, kcc

Differential Revision: http://reviews.llvm.org/D16419

llvm-svn: 258473