design document for a hardware-assisted memory safety (HWAMS) tool, similar to AddressSanitizer

Summary:
preliminary design document for a hardware-assisted memory safety (HWAMS) tool, similar to AddressSanitizer
The name TaggedAddressSanitizer and the rest of the document, are early draft, suggestions are welcome.

The code will follow shortly.

Reviewers: eugenis, alekseyshl

Reviewed By: eugenis

Subscribers: davidxl, cryptoad, fedor.sergeev, cfe-commits, llvm-commits

Differential Revision: https://reviews.llvm.org/D40568

llvm-svn: 319684