[lldb][split-dwarf] implement GetSeparateDebugInfo for SymbolFileOnDemand (#71230)

Small change to get `image dump separate-debug-info` working when using
`symbols.load-on-demand`.

Added tests to `TestDumpDwo`, and enabled the test for all platforms. If we fail to build, we skip the test, so this shouldn't cause the test to fail on unsupported platforms.
```
bin/lldb-dotest -p TestDumpDwo
```

It's easy to verify this manually by running 
```
lldb --one-line-before-file "settings set symbols.load-on-demand true" <some_target>
(lldb) image dump separate-debug-info
...
```

---------

Co-authored-by: Tom Yang <toyang@fb.com>