Hardware-assisted AddressSanitizer (compiler-rt)

Summary:
Runtime library for HWASan, initial commit.
Does not randomize tags yet, does not handle stack or globals.

Reviewers: kcc, pcc, alekseyshl

Subscribers: srhines, kubamracek, dberris, mgorny, llvm-commits, krytarowski

Differential Revision: https://reviews.llvm.org/D40935

llvm-svn: 320231