This patch updates LTOCodeGenerator to use the utilities provided by
LTOBackend to run middle-end optimizations and backend code generation.
This is a first step towards unifying the code used by libLTO's C API
and the newer, C++ interface (see PR41541).
The immediate motivation is to allow using the new pass manager when
doing LTO using libLTO's C API, which is used on Darwin, among others.
This is still WIP, as some of the error handling still needs to be
updated/improved, but I wanted to share it early, to get some more
Also, some options are not yet supported via lto::Config.
With the changes, there are no codegen/stats differences when building
MultiSource/SPEC2000/SPEC2006 on X86 with LTO, compared to without the