We noticed that when invoking the thinBackend via clang (for the
distributed build case) that flags like -ffunction-sections and
-emit-llvm were not having the intended effect. This could have been
fixed by setting up the TargetOptions and the CodeGenFileType in the LTO
Config, but since clang already has handling for all of this, it is
straightforward to just let it do the handling (just below the
early return after runThinLTOBackend being removed here).
Depends on D31100.