[LLDB][PDB] Access object file through module (#169728)
When a PDB is loaded through `target symbols add <pdb-path>`, its
`m_objectfile_sp` is an `ObjectFilePDB` instead of `ObjectFilePECOFF`
(the debugged module). In both the native and DIA plugin, some paths
assumed that `m_objectfile_sp` is the debugged module. With this PR,
they go through `m_objfile_sp->GetModule()->GetObjectFile()`.
For the DIA plugin, this lead to an assertion failure
(https://github.com/llvm/llvm-project/issues/169628#issuecomment-3582555277)
and for both plugins, it meant that the symbol table wasn't loaded.
