[WebAssembly] Support for atomic.wait / atomic.wake builtins

Summary:
Add support for atomic.wait / atomic.wake builtins based on the Wasm
thread proposal.

Reviewers: dschuff

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

Differential Revision: https://reviews.llvm.org/D49396

llvm-svn: 338771