[ELF] Explicit template instantiations for addFile

Summary:
All other templated methods have explicit instantiations but this one is
missing. Discovered while building with a clang with inliner
modifications.

Reviewers: espindola

Subscribers: emaste, llvm-commits, davidxl

Differential Revision: https://reviews.llvm.org/D41847

llvm-svn: 322057