Summary: - lowers @llvm.global_dtors by adding @llvm.global_ctors functions which register the destructors with `__cxa_atexit`. - impements @llvm.global_ctors with wasm start functions and linker metadata See [here](https://github.com/WebAssembly/tool-conventions/issues/25) for more background. Subscribers: jfb, dschuff, mgorny, jgravelle-google, aheejin, sunfish Differential Revision: https://reviews.llvm.org/D41211 git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@320774 91177308-0d34-0410-b5e6-96231b3b80d8