Add a helper method for collecting line table file references that
does the correct thing for DWARF 5 vs DWARF 4 (in the latter case you
have an implicit 0 entry which is the comp dir, whereas in the former
case you do not). This is to avoid out-of-bounds errors when examining
the file table section of a DWARF 5 compilation unit's line table.
Included is a new linux/amd-only test that includes a precompiled C
object file with a DWARF-5 section that triggers the bug in question.
Fixes#2319
