[DebugInfo] Parse StrOffsets section if needed

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).

Differential Revision: https://reviews.llvm.org/D159484