[lld] Prevent duplicate files in debug line header in dwarf 5.

Summary:
Changes to a couple of tests that will start failing after https://reviews.llvm.org/D59515 is submitted.

Reviewers: echristo, ruiu, espindola

Reviewed By: echristo, ruiu

Subscribers: emaste, arichardson, MaskRay, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D59671

llvm-svn: 356943