DebugInfo: Fix incorrect line table lookup when resolving decl_file from a split unit

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)