llvm-dwarfdump --verify: Add support for .debug_str_offsets[.dwo]

Had a couple of issues lately causing corrupted strings due to
problematic str_offsets (overflow due to >4GB .debug_str.dwo section in
a dwp and the dwp tool silently overflowing the 32 bit offsets updated
in the .debug_str_offsets.dwo section, and then more recently two CUs in
a dwo caused the dwp tool to reapply the offset adjustment twice
corrupting str_offsets.dwo as well) - so let's check that the offsets
are valid.

This assumes no suffix merging - if anyone implements that, then this
checking should just be removed for the most part (we could still check
the offsets are within the bounds of .debug_str[.dwo], but nothing more
- any offset in the range would be valid, the offsets wouldn't have to
land at the start of a string)