[WebAssembly] Change wasm.throw's first argument to an immediate

Summary:
`wasm.throw` builtin's first 'tag' argument should be an immediate index
into the event section.

Reviewers: dschuff, craig.topper

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D59448

llvm-svn: 356436