[llvm-profdata] Add SaturatingAdd/SaturatingMultiply Helper Functions (2nd try)

Summary:
This change adds MathExtras helper functions for handling unsigned, saturating addition and multiplication. It also updates the instrumentation and sample profile merge implementations to use them.

Reviewers: dnovillo, bogner, davidxl

Subscribers: davidxl, llvm-commits

Differential Revision: http://reviews.llvm.org/D14720

llvm-svn: 253497