The aim is to include just (or primarily) code generation affecting options
explicitly passed on the command line. So that the merging actually works,
options or arguments which include filenames or paths shouldn't be added,
on Roland's request -D*/-U* options aren't added either (that should be
covered by .debug_macinfo)
Ideally we'd just include explicitly passed options from command line that
haven't been overridden by other command line options, and would sort them,
so that there are higher chances of DW_AT_producer strings being merged
(e.g. -O2 -ffast-math vs. -ffast-math -O2 are now different strings, and
similarly -O2 vs. -O3 -O2 vs. -O0 -O1 -Ofast -O2), but I'm not sure if it is
easily possible using current option handling framework.