[lldb] Add --changed option to `settings show` (#196390)
Add a `--changed`/`-c` flag to `settings show` that restricts the output
to settings whose current value differs from the default. This makes it
easy to inspect what has been customized in a session or config without
scrolling through the full property tree.
One thing worth calling out is that this works as expected with explicit
property paths, for example you can show only the modified settings
belonging to `target`:
```
(lldb) set show -c target
target.load-script-from-symbol-file (enum) = true (default: trusted)
(lldb)
```
If nothing has been changed, the output is empty:
```
(lldb) sett show -c target.process
(lldb)
```
rdar://176483441
