The goal is to deprecate the existing C API exposed by libLTO and
remove it 2 or 3 releases from now.
This commit introduces the basic replacement APIs, and it not intended
to be a complete replacement at this point.
A new tool is introduced: llvm-liblto. This tool links statically
only to libSupport, and link dynamically to libLTO. It offers a way
to test the new C API in tree without having a linker using this API
available. This was a major painpoint of the existing API.
This is a work-in-progress, I'm sharing it now to show the general
direction I took until now.