[WebAssembly] Bulk memory intrinsics and builtins

Summary:
implements llvm intrinsics and clang intrinsics for
memory.init and data.drop.

Reviewers: aheejin

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D57736

llvm-svn: 353983