[DWARFv5] Emit file 0 to the line table.

DWARF v5 specifies that the root file (also given in the DW_AT_name
attribute of the compilation unit DIE) should be emitted explicitly to
the line table's list of files.  This makes the line table more
independent of the .debug_info section.

Differential Revision: https://reviews.llvm.org/D44054

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@326758 91177308-0d34-0410-b5e6-96231b3b80d8