Reset tree tracking when scopes response received

[colin-grant-work]

What it does

The code for tracking variables would sometimes report variables multiple times if the variablesReference value for a given scope (e.g. 'Locals') changed.

This PR adjusts the handling to clear the tree when the response to the scopes request is received: that request implies that all scopes will be refreshed (possible with different variablesReference values), so that whatever old data is available locally is (possibly) invalid.

NB: we'll have some work coming in from @WyoTwT that will make the variable retrieval for Memory Inspector views independent of the routine fetching for a debug session.

How to test

1. Debug a program (e.g. with the CDT GDB adapter)
2. Load the memory view with variables visible.
3. Step a number of times (e.g. through a loop)
4. Observe that a given variable only appears once in the variables column.

Review checklist

• as an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• as a reviewer, I agree to behave in accordance with the review guidelines

[jreineckearm]

Change looks good and makes sense.
Tested with Arm Debugger debug adapter and works as expected.

[planger]

Thank you, that makes sense! Tested with CDT GDB Adapter for a simple CMake project.