[WebAssembly] Add support for init functions linking metadata
Summary:
This change lays the groundwork lowering of @llvm.global_ctors
and @llvm.global_dtors for the wasm object format. Some parts
of this patch are subset of: https://reviews.llvm.org/D40759
See https://github.com/WebAssembly/tool-conventions/issues/25
Subscribers: jfb, dschuff, jgravelle-google, aheejin, sunfish
Differential Revision: https://reviews.llvm.org/D41208
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@320742 91177308-0d34-0410-b5e6-96231b3b80d8