Turn LLVM modules into a binary

[jonmeow]

Capturing some notes from @chandlerc:

There are two steps here

1. converting to a .o file
2. linking together the .o files into an executable
   for (2) I think you'll want to use the Clang driver to run a link subprocess (which will use lld or some other linker on the system)
   for (1), you'll want to do this directly. llc does do this, but I wouldn't use it as a reference. I would use the code in Clang in BackendUtil.cpp
   https://github.com/llvm/llvm-project/blob/main/clang/lib/CodeGen/BackendUtil.cpp
   None of that file's routines are really in usefully public and reliably APIs
   I wouldn't depend on it
   I would look at how it uses the LLVM APIs, and then directly use the LLVM APIs to emit the .o file
   (this file is considered an implementation detail of Clang)
   (but it's usage of the LLVM APIs is likely a better reference that llc's usage of LLVM APIs)

Per some side-discussion, most of the BackendUtil.cpp won't be used, e.g. ThinLTO. We might want that eventually but not right now, and possibly rewritten. Rather, this really is for the most core part of things like using the pass manager and the core LLVM APIs for .o.

(note, not working on this right now, but taking this down to remember the discussion)