We need to attach the same information to DkmEvaluationResults and
DkmEvaluationResultEnumContexts, but they have different lifetimes.  Enum
contexts, which are, in some sense, the children of evaluation results are
closed first.  As such we don't want to clean up our attached information
until the evaluation result is closed.  When we are informed that this has
happened (by a call to EvalResultDataItem.OnClose), we call Close on the
associated DkmClrValue.

We hope that actively managing the lifetime of DkmClrValue instances will
improve perf by reducing the amount of work that the debugger has to do on
each evaluation (which is proportional to the number of live
DkmClrValues).

Unfortunately, this will not improve perf in the large-array case because
DkmEvaluationResults are not closed as they are scrolled out of view.

Associated with DevDiv #1094428 (not yet on GitHub).