[LTO] Use lto::backend for code generation.

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.

With the changes, there are no codegen/stats differences when building
MultiSource/SPEC2000/SPEC2006 on Darwin X86 with LTO, compared
to without the patch.

Reviewed By: steven_wu

Differential Revision: https://reviews.llvm.org/D94487