[DebugInfo][DWARF64] Fix dumping pre-standard .debug_str_offsets.dwo sections.

The sizes of offsets in the `.debug_str_offsets.dwo` section depend on
the format of compilation or type units referencing them: 4 bytes for
DWARF32 units and 8 bytes for DWARF64 ones. The fix uses parsed units
to determine the actual size of offsets in the corresponding part of
the `.debug_str_offsets.dwo` section.

Differential Revision: https://reviews.llvm.org/D78555