[Support] Change SaturatingAdd()/SaturatingMultiply() to use pointer for returning overflow state

Summary:
Improve SaturatingAdd()/SaturatingMultiply() to use bool * to optionally return overflow result.
This should make it clearer that the value is returned at callsites and reduces the size of the implementation.

Reviewers: davidxl, silvas

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D15219

llvm-svn: 255128