Hardware-assisted AddressSanitizer (clang part).

Summary:
Driver, frontend and LLVM codegen for HWASan.
A clone of ASan, basically.

Reviewers: kcc, pcc, alekseyshl

Subscribers: srhines, javed.absar, cfe-commits

Differential Revision: https://reviews.llvm.org/D40936

llvm-svn: 320232