Sanitizers.h - remove MathExtras.h include dependency

The MathExtras.h header is included purely for the countPopulation() method - by moving this into Sanitizers.cpp we can remove the use of this costly header.

We only ever use isPowerOf2() / countPopulation() inside asserts so this shouldn't have any performance effects on production code.

Differential Revision: https://reviews.llvm.org/D103953