[analyzer][NFC] Use std::optional instead of custom "empty" state

This commit eliminates the uninitialized error state from the class
RegionRawOffsetV2 (which is locally used by the Clang Static Analyzer
checker alpha.security.ArrayBoundV2) and replaces its use with
std::optional.

Motivated by https://reviews.llvm.org/D148355#inline-1437928

Moreover, the code of RegionRawOffsetV2::computeOffset() is rearranged
to clarify its behavior. The helper function getValue() was eliminated
by picking a better initial value for the variable Offset; two other
helper functions were replaced by the lambda function Calc() because
this way it doesn't need to take the "context" objects as parameters.

This reorganization revealed some surprising (but not outright buggy)
behavior that's marked by a FIXME and will be revisited in a separate
commit.

Differential Revision: https://reviews.llvm.org/D149259