[DWARFVerifier] Fix debug_str_offsets DWARF version detection (#81303)

The DWARF 5 debug_str_offsets section starts with a header, which must
be skipped in order to access the underlying `strp`s.

However, the verifier supports some pre-standardization version of this
section (with the same section name), which does not have a header. In
this case, the offsets start on the first byte of the section. More in
[1] and [2] about this legacy section.

How does The DWARF verifier figure out which version to use? It manually
reads the **first** header in debug_info and uses that. This is wrong
when multiple debug_str_offset sections have been linked together, in
particular it is wrong in the following two cases:

1. A standard DWARF 4 object file (i.e. no debug_str_offsets) linked
with a standard DWARF 5 object file.
2. A non-standard DWARF 4 object file (i.e. containing the header-less
debug_str_offsets section) linked with a standard DWARF 5 object file.

Based on discussions in https://github.com/llvm/llvm-project/pull/81210,
the legacy version is only possible with dwo files, and dwo files cannot
mix the legacy version with the dwarf 5 version. As such, we change the
verifier to only check the debug_info header in the case of dwo files.
If it sees a dwarf 4 version, it handles it the legacy way.

Note: the modified test was technically testing an unsupported
combination of dwarf version + non-dwo sections. To see why, simply note
that the test contained no `debug_info.dwo` sections, so the call to
DWARFObject::forEachInfoDWOSections was doing nothing. We were finding
the error through the "standard version", which shouldn't happen.

[1]: https://gcc.gnu.org/wiki/DebugFission 
[2]: https://gcc.gnu.org/wiki/DebugFissionDWP