Querying the debug_str_offsets section requires parsing the top level DIE of the
CU (as well as the section itself); the current getter, however, assumes this is
done elsewhere. This patch changes the getter behavior to match what is done in
other getter methods (e.g. getCompilationDir or getVariableForAddress), in
other words, extractDIEsIfNeeded is now called prior to returning the
debug_str_offsets contributions for the Unit.
One way in which this bug manifested is when dwarfdump --debug-str-offsets is
invoked: because the DIEs are never parsed, we incorrectly print an empty
section (with no warnings or errors).