[stackprotector] Refactor the StackProtector pass from a single .cpp file into StackProtector.h and StackProtector.cpp.

No functionality change.  Future patches will add analysis which will be used
in other passes (PEI, StackSlot).  The end goal is to support ssp-strong stack
layout rules.

WIP.

Differential Revision: http://llvm-reviews.chandlerc.com/D1521