DebugInfo: Don't include the name of the CU file in the line table file list when it's unneeded

This comes up in empty files or files containing #file directives that
never reference the actual source file name. Came up in a small test of
line tables I was playing with.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@201187 91177308-0d34-0410-b5e6-96231b3b80d8