Debugger: Further improvements to expression handling.

VariablesView:
- ModelNode now acquires a reference to its target variable. This
  is necessary because in the case of expression variables, the
  model node is ultimately the only one aware of it, and therefore
  needs to manage its lifetime.
- AddSyntheticNode() now discriminates between the case where it
  needs to create a node child on behalf of the caller versus when
  it's given an already created one, as is the case when an expression
  resolves to a value node. Consequently it also detects whether
  it needs to request value resolution or not, and handles accordingly.
- _AddExpressionNode() now supports value node results and handles
  them accordingly.