DebugInfo: Remove dependence on file numbering in the line table.

These tests were unnecessarily sensitive to the presence and ordering of
elements in the line table file_names list which will break on a future
change I'm working on.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@201185 91177308-0d34-0410-b5e6-96231b3b80d8