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

Reapply rL356943; the previous attempt was reverted because the patch rL356941
that this depended on had broken a test.

Original commit message:

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

Tags: #llvm

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

llvm-svn: 357020