[DebugInfo] Don't join DW_AT_comp_dir and directories[0] for DWARF v5 line tables

DWARF v5 6.2.4 The Line Number Program Header says:

> The first entry is the current directory of the compilation. Each additional
> path entry is either a full path name or is relative to the current directory of
> the compilation.

When forming a path, relative DW_AT_comp_dir and directories[0] are not supposed
to be joined together. Fix getFileNameByIndex to special case DWARF v5 DirIdx == 0.

Reviewed By: #debug-info, dblaikie

Differential Revision: https://reviews.llvm.org/D131804