[clang][dataflow] Improve optional model's support for ignoring smart pointers.

The optional model has an option to ignore optionals accessed through smart
pointer types (other than optional itself). This patch improves this feature in
two ways:

1. We extend support to optionals accessed directly through the smart pointer,
like `ptr->value()`. Previously, support was limited to cases that went through
an intermediate field.

2. We clean up the implementation by removing the option from the analysis,
leaving it only in the diagnostic phase (where it is relevant).

3. Adjusts a test which was misleading in what it was testing.

Differential Revision: https://reviews.llvm.org/D140020