[lldb] Fix data buffer regression in ObjectFile (#177724)
This fixes a regression in `ObjectFile` and `ObjectFileELF` introduced
by #171574.
The original code created a `DataBuffer` using `MapFileDataWritable`.
```
data_sp = MapFileDataWritable(*file, length, file_offset);
if (!data_sp)
return nullptr;
data_offset = 0;
```
The new code requires converting the `DataBuffer` to a `DataExtractor`:
```
DataBufferSP buffer_sp = MapFileDataWritable(*file, length, file_offset);
if (!buffer_sp)
return nullptr;
extractor_sp = std::make_shared<DataExtractor>();
extractor_sp->SetData(buffer_sp, data_offset, buffer_sp->GetByteSize());
data_offset = 0;
```
The issue is that once we get a data buffer back from
MapFileDataWritable, we don't have to adjust for the `data_offset` again
when calling `SetData` as the `DataBuffer` is already normalized to have
a zero start offset. A similar issue exists in `ObjectFile`.
rdar://168317174
