Internal expressions shouldn't increment the result variable numbering.

Internal expressions shouldn't increment the result variable numbering.

There an option: EvaluateExpressionOptions::SetResultIsInternal to indicate
whether the result number should be returned to the pool or not.  It
got broken when the PersistentExpressionState was refactored.

This fixes the issue and provides a test of the behavior.

Differential Revision: https://reviews.llvm.org/D76532