[libFuzzer] Use std::discrete_distribution for input selection.

Summary:
Since we're casting from double to size_t during input selection, we
really want a discrete distribution over size_t rather than a piecewise
distribution over doubles.

Reviewers: kcc

Reviewed By: kcc

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D50356

llvm-svn: 339973