[clang][dataflow][NFC] Fix reachability warning.

[ymandel]

Summary

While cases of the switch return (or are unreachable), if a non-enum integer was cast into the enum, it's possible for this switch to fall through without returning, hence allowing control to reach the end of a non-void function. So, we explicitly mark with llvm_unreachable after the switch.

[xazax.hun]

I'd argue the warning is correct as you could store "non-enum" integer values into an enum typed variable (and people do that fairly often with bitwise enums).

[ymandel]

In D135978#3859272, @xazax.hun wrote:

I'd argue the warning is correct as you could store "non-enum" integer values into an enum typed variable (and people do that fairly often with bitwise enums).

Good point. Feels wrong, but is perfectly legal in C++. :(

clang/lib/Analysis/FlowSensitive/Transfer.cpp

return unpackBinaryBoolValue<DisjunctionValue>(Env, V,
                                                    &Environment::makeOr);
   case Value::Kind::Implication:
     return unpackBinaryBoolValue<ImplicationValue>(
         Env, V, &Environment::makeImplication);
   case Value::Kind::Biconditional:
     return unpackBinaryBoolValue<BiconditionalValue>(Env, V,
                                                      &Environment::makeIff);
   }
+  llvm_unreachable("All reachable cases in switch return");
 }
 
 // Unpacks the value (if any) associated with `E` and updates `E` to the new
 // value, if any unpacking occured.
 static Value *maybeUnpackLValueExpr(const Expr &E, Environment &Env) {
   auto *Loc = Env.getStorageLocation(E, SkipPast::Reference);
   if (Loc == nullptr)
     return nullptr;