[LTO] Make processing of combined module more consistent

Summary:
1. Use stream 0 only for combined module. Previously if combined module was not
processes ThinLTO used the stream for own output. However small changes in input,
could trigger combined module  and shuffle outputs making life of llvm::LTO harder.

2. Always process combined module and write output to stream 0. Processing empty
combined module is cheap and allows llvm::LTO users to avoid implementing processing
which is already done in llvm::LTO.

Subscribers: mehdi_amini, inglorion, eraman, hiraditya

Differential Revision: https://reviews.llvm.org/D41267

llvm-svn: 320905