[WebAssembly] Allow multivalue signatures in object files

Also changes the wasm YAML format to reflect the possibility of having
multiple return types and to put the returns after the params for
consistency with the binary encoding.

Reviewers: aheejin, sbc100

Subscribers: dschuff, jgravelle-google, hiraditya, sunfish, arphaman, rupprecht, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D69156

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@375283 91177308-0d34-0410-b5e6-96231b3b80d8
35 files changed