[LTO] Add a new splitCodeGen() API which takes a TargetMachineFactory.

This will be used in lld to avoid creating TargetMachine in two
different places. See D18999 for a more detailed discussion.

Differential Revision:  http://reviews.llvm.org/D19139

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@266390 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed