[clang][dataflow][NFC] Add a FIXME to handling of union initialization. (#82239)

We want to make it clear that the current behavior doesn't yet handle
unions
properly.

Author: martinboehme

clang/lib/Analysis/FlowSensitive/Transfer.cpp

@@ -664,6 +664,7 @@ class TransferVisitor : public ConstStmtVisitor<TransferVisitor> {
     QualType Type = S->getType();
 
     if (Type->isUnionType()) {
+      // FIXME: Initialize unions properly.
       if (auto *Val = Env.createValue(Type))
         Env.setValue(*S, *Val);
       return;