[clang] [clang][dataflow]Use cast_or_null instead cast to prevent crash (PR #68510)

[Yitzhak Mandelbaum via cfe-commits]

================
@@ -180,3 +180,44 @@ void std_forward_rvalue_ref_safe(absl::optional<int>&& opt) {
 
   std::forward<absl::optional<int>>(opt).value();
 }
+
+namespace std {
+
+template<typename T>
+class optional {
+public:
+	template <typename U>
+  optional& operator=(const U &u){
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ymand wrote:

The issue isn't specifix to `std::optional`. Please just use the `absl` version we already include, rather than adding another optional definition.

https://github.com/llvm/llvm-project/pull/68510