DWARF v5: fix directory index in the line table

Summary:
Prior to DWARF v5, a directory index of 0 represents DW_AT_comp_dir.

In DWARF v5, the index starts with 0 and Entry.DirIdx is the index into
Prologue.IncludeDirectories.

Reviewed By: labath

Differential Revision: https://reviews.llvm.org/D61253

llvm-svn: 360015