[Reland] Report only loaded debug info in statistics dump (#81706) (#82207)

Updates:

- The previous patch changed the default behavior to not load dwos in
`DWARFUnit`
~~`SymbolFileDWARFDwo *GetDwoSymbolFile(bool load_all_debug_info =
false);`~~
`SymbolFileDWARFDwo *GetDwoSymbolFile(bool load_all_debug_info = true);`
- This broke some lldb-shell tests (see
https://green.lab.llvm.org/green/view/LLDB/job/as-lldb-cmake/16273/)
- TestDebugInfoSize.py
- with symbol on-demand, by default statistics dump only reports
skeleton debug info size
- `statistics dump -f` will load all dwos. debug info = skeleton debug
info + all dwo debug info

Currently running `statistics dump` will trigger lldb to load debug info
that's not yet loaded (eg. dwo files). Resulted in a delay in the
command return, which, can be interrupting.

This patch also added a new option `--load-all-debug-info` asking
statistics to dump all possible debug info, which will force loading all
debug info available if not yet loaded.