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
Specifically in DWARFv5 the unit for the line table entry was correct
but the context was incorrect - leading to looking up .debug_line_str in
the dwp instead of the executable.
(perhaps we could/should remove the context pointer entirely, and rely
on the one in the unit... I might try that as a separate follow-up
commit)
Fangrui Song