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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@299152 91177308-0d34-0410-b5e6-96231b3b80d8