[clang] [Clang] Add __builtin_is_within_lifetime to implement P2641R4's std::is_within_lifetime (PR #91895)

[via cfe-commits]

================
@@ -17160,3 +17184,72 @@ bool Expr::tryEvaluateStrLen(uint64_t &Result, ASTContext &Ctx) const {
   EvalInfo Info(Ctx, Status, EvalInfo::EM_ConstantFold);
   return EvaluateBuiltinStrLen(this, Result, Info);
 }
+
+namespace {
+struct IsWithinLifetimeHandler {
+  EvalInfo &Info;
+  static constexpr AccessKinds AccessKind = AccessKinds::AK_IsWithinLifetime;
+  using result_type = std::optional<bool>;
+  std::optional<bool> failed() { return std::nullopt; }
+  template <typename T>
+  std::optional<bool> found(T &Subobj, QualType SubobjType) {
+    return true;
+  }
+};
+
+std::optional<bool> EvaluateBuiltinIsWithinLifetime(IntExprEvaluator &IEE,
+                                                    const CallExpr *E) {
+  EvalInfo &Info = IEE.Info;
+  // assert(Info.InConstantContext && "Call to consteval builtin not in constant
+  // context?");
+  assert(E->getBuiltinCallee() == Builtin::BI__builtin_is_within_lifetime);
----------------
cor3ntin wrote:

Why is that commented? It certainly looks like something that should not be constant folded

https://github.com/llvm/llvm-project/pull/91895