[lldb-dap] Add readOnly attribute for variables (#151884)
This patch adds `readOnly`
[attribute](https://microsoft.github.io/debug-adapter-protocol/specification#Types_VariablePresentationHint)
for variables. When this attribute is returned for a variable, VS Code
prevents editing (and grays out the `Set Value` button). Without this,
users might be confused if the UI allows edits but the debug adapter
often returns an error. I checked `SetValueFromCString` function
implementation and found that it doesn't support aggregate data types,
so I added simple check for them. Also, I found that I can update value
of registers sets (but without any effects). So I also added checks for
those as well.
