[analyzer][NFC] Change return value of StoreManager::attemptDownCast function...

[analyzer][NFC] Change return value of StoreManager::attemptDownCast function from SVal to Optional<SVal>

Summary: Refactor return value of `StoreManager::attemptDownCast` function by removing the last parameter `bool &Failed` and replace the return value `SVal` with `Optional<SVal>`.  Make the function consistent with the family of `evalDerivedToBase` by renaming it to `evalBaseToDerived`. Aligned the code on the call side with these changes.

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