[ThinLTO] Set up lto::Config properly for codegen in ThinLTO backends

Summary:
This involved refactoring out pieces of
EmitAssemblyHelper::CreateTargetMachine for use in runThinLTOBackend.

Subsumes D31114.

Reviewers: mehdi_amini, pcc

Subscribers: Prazek, cfe-commits

Differential Revision: https://reviews.llvm.org/D31508

llvm-svn: 299152