[LTO] Add the ability to test -thinlto-emit-imports-files through llvm-lto2

Summary:
Start bringing llvm-lto2 to a level where we can test the LTO API
a bit deeper.

Reviewers: tejohnson

Subscribers: llvm-commits, mehdi_amini

Differential Revision: https://reviews.llvm.org/D23681

llvm-svn: 279349