A more robust way of testing debug_line parser near the end of module

Summary:
While removing -z separate-code makes lld produce place the code at the
end of a segment right now, it's possible that future changes to the
linker will change that, thereby removing the coverage for the changes
in r367983. This patch adds a linker script to one of the line table
tests, which ensures that the code (and its line table) will be placed
at the very end of a module.

Reviewers: MaskRay

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D65789

llvm-svn: 368154