[vsdebug] "Invalid variable attributes" error when trying to inspect variables during debugging

**Describe the bug**
VS Code debugger shows "Invalid variable attributes" in the variables panel when stopped at a breakpoint.

This is happening because the Debug Adapter Protocol requires a `variablesReference` field in the `Variable` type sent by the debug server (https://microsoft.github.io/debug-adapter-protocol/specification#Types_Variable), which HHVM currently doesn't return. It is likely that VS Code started enforcing this field in a recent version, which led to this behavior.

I have fixed it on the VS Code extension side by adding an override to the "variables" response (https://github.com/slackhq/vscode-hack/pull/92), but it should ideally be fixed on the debug server side.