[LTO] Call InitLLVM from llvm-lto2

Summary:
D45602 added this to most tools, including llvm-lto, but not to
llvm-lto2. Add it there and test that it works in both lto tools.

Reviewers: ruiu

Subscribers: mehdi_amini, inglorion, eraman, steven_wu, dexonsmith, llvm-commits

Differential Revision: https://reviews.llvm.org/D53330

llvm-svn: 344631