[WebAssembly] Support Leak Sanitizer on Emscripten

Summary:
LSan is currently being ported to Emscripten and mostly works.

Enabling the support in upstream would simplify testing.

Patch by Guanzhong Chen.

Reviewers: tlively, aheejin

Reviewed By: aheejin

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D62830

llvm-svn: 362667