[clang][dataflow] Weaken guard to only check for storage location

Weaken the guard for whether a sub-expression has been evaluated to
only check for the storage location, instead of checking for the
value. It should be sufficient to check for the storage location, as
we don't necessarily guarantee that a value will be set for the
location (although this is currently true right now).

Differential Revision: https://reviews.llvm.org/D125823