[WebAssembly] Support VPtr sanitizer for Emscripten

[WebAssembly] Support VPtr sanitizer for Emscripten

Summary:
After https://github.com/emscripten-core/emscripten/pull/8651, Emscripten
supports the full UBSan runtime. This includes the VPtr sanitizer.

This diff allows clang to generate code that uses the VPtr sanitizer for
Emscripten.

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/D62559

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@362004 91177308-0d34-0410-b5e6-96231b3b80d8