[clang][dataflow] In `optional` model, match call return via hasType

Currently the unchecked-optional-access model fails on this example:

    #include <memory>
    #include <optional>

    void foo() {
      std::unique_ptr<std::optional<float>> x;
      *x = std::nullopt;
    }

You can verify the failure by saving the file as `foo.cpp` and running this command:

    clang-tidy -checks='-*,bugprone-unchecked-optional-access' foo.cpp -- -std=c++17

The failing `assert` is in the `transferAssignment` function of the `UncheckedOptionalAccessModel.cpp` file:

    assert(OptionalLoc != nullptr);

The symptom can be treated by replacing that `assert` with an early `return`:

    if (OptionalLoc == nullptr)
      return;

That would be better anyway since we cannot expect to always cover all possible LHS expressions, but it is out of scope for this patch and left as a followup.

Note that the failure did not occur on this very similar example:

    #include <optional>

    template <typename T>
    struct smart_ptr {
      T& operator*() &;
      T* operator->();
    };

    void foo() {
      smart_ptr<std::optional<float>> x;
      *x = std::nullopt;
    }

The difference is caused by the `isCallReturningOptional` matcher, which was previously checking the `functionDecl` of the `callee`. This patch changes it to instead use `hasType` directly on the call expression, fixing the failure for the `std::unique_ptr` example above.

Reviewed By: sgatev

Differential Revision: https://reviews.llvm.org/D127434