[WebAssembly] Allow multivalue signatures in object files

Summary:
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