[lldb-dap] Updating VariableDescription to use GetDescription() as a fallback. (#77026)

When generating a `display_value` for a variable the current approach
calls `SBValue::GetValue()` and `SBValue::GetSummary()` to generate a
`display_value` for the `SBValue`. However, there are cases where both
of these return an empty string and the fallback is to print a pointer
and type name instead (e.g. `FooBarType @ 0x00321`).

For swift types, lldb includes a langauge runtime plugin that can
generate a description of the object but this is only used with
`SBValue::GetDescription()`.

For example:
```
$ lldb swift-binary
... stop at breakpoint ...
lldb> script
>>> event = lldb.frame.GetValueForVariablePath("event")
>>> print("Value", event.GetValue())
Value None
>>> print("Summary", event.GetSummary())
Summary None
>>> print("Description", event) # __str__ calls SBValue::GetDescription()
Description (main.Event) event = (name = "Greetings", time = 2024-01-04 23:38:06 UTC)
```

With this change, if GetValue and GetSummary return empty then we try
`SBValue::GetDescription()` as a fallback before using the previous
logic of printing `<type> @ <addr>`.