[clang][dataflow] Don't `assert` full LHS coverage in `optional` model

[samestep]

Summary

Followup to D127434.

[ymandel]

Do I understand correctly that the test you added in D127434 covers this change as well? (in that this change moves it from crashing to not crashing)

[samestep]

@ymandel Yes, either this patch or D127434 would bring that test from crashing to passing.

clang/lib/Analysis/FlowSensitive/Models/UncheckedOptionalAccessModel.cpp

 }
 
 void transferAssignment(const CXXOperatorCallExpr *E, BoolValue &HasValueVal,
                         LatticeTransferState &State) {
   assert(E->getNumArgs() > 0);
 
   auto *OptionalLoc =
       State.Env.getStorageLocation(*E->getArg(0), SkipPast::Reference);
-  assert(OptionalLoc != nullptr);
+  if (OptionalLoc == nullptr)
+    return;
 
   State.Env.setValue(*OptionalLoc, createOptionalValue(State.Env, HasValueVal));
 
   // Assign a storage location for the whole expression.
   State.Env.setStorageLocation(*E, *OptionalLoc);
 }
 
 void transferValueOrConversionAssignment(