Dwarf stores source-file names the three parts:
Prior to this change, the code only allowed retrieving either all
three as the absolute path, or just the filename. But many
compile-command lines--especially those in hermetic build systems
don't specify an absolute path, nor just the filename, but rather the
path relative to the compilation directory. This features allows
retrieving them in that style.
Not sure of the best place for a test for this. Neither of the
other options are tested explicitly. The unittests are all very
low level (Can we read .debug_info properly? And a symbolizer
test would require adding a new flag to the symbolizer.