[lldb-vscode] Show a fake child with the raw value of synthetic types (#65552)

Currently, if the user wants to inspect the raw version of a synthetic
variable, they have to go to the debug console and type `frame var
<variable>`, which is not a great experience. Taking inspiration from
CodeLLDB, this adds a `[raw]` child to every synthetic variable so that
this kind of inspection can be done visually.

Some examples:

<img width="500" alt="Screenshot 2023-09-06 at 7 56 25 PM"
src="https://github.com/llvm/llvm-project/assets/1613874/7fefb7c5-0da7-49c7-968b-78ac88348fea">
<img width="479" alt="Screenshot 2023-09-06 at 6 58 25 PM"
src="https://github.com/llvm/llvm-project/assets/1613874/6e650567-16e1-462f-9bf5-4a3a605cf6fc">