[clang][dataflow] Relax validity assumptions in `UncheckedOptionalAccessModel`.

Currently, the interpretation of `swap` calls in the optional model assumes the
optional arguments are modeled (and therefore have valid storage locations and
values). This assumption is incorrect, for example, in the case of unmodeled
optional fields (which can be missing either value or location). This patch
relaxes these assumptions, to return rather than assert when either argument is
not modeled.

Differential Revision: https://reviews.llvm.org/D142710