[WebAssembly] Implement Address Sanitizer for Emscripten

Summary:
This diff enables address sanitizer on Emscripten.

On Emscripten, real memory starts at the value passed to --global-base.

All memory before this is used as shadow memory, and thus the shadow mapping
function is simply dividing by 8.

Reviewers: tlively, aheejin, sbc100

Reviewed By: sbc100

Subscribers: dschuff, sbc100, jgravelle-google, hiraditya, sunfish, cfe-commits, llvm-commits

Tags: #clang, #llvm

Differential Revision: https://reviews.llvm.org/D63742

llvm-svn: 364468