[Support] Fix SaturatingMultiply<T>() to be correct (and fast), Re-enable Unit Tests

Summary:
This change fixes the SaturatingMultiply<T>() function template to not cause undefined behavior with T=uint16_t.
Thanks to Richard Smith's contribution, it also no longer requires an integer division.

Patch by Richard Smith.

Reviewers: silvas, davidxl

Subscribers: rsmith, davidxl, llvm-commits

Differential Revision: http://reviews.llvm.org/D14845

llvm-svn: 253870