[clang][dataflow] Simplify handling of nullopt-optionals.

Previously, in the case of an optional constructed from `nullopt`, we relied on
the value constructed for the `nullopt`. This complicates the implementation and
exposes it to bugs (indeed, one such was found), yet doesn't improve the
engine. Instead, this patch constructs a fresh optional representation, rather
than relying on the underlying nullopt representation.

Differential Revision: https://reviews.llvm.org/D140506