[flang][NFC] static assert intrinsic table is sorted (#120399)

This invariant is used below when searching for intrinsic
implementation. Currently, if the map is not sorted, the compiler will
just silently assume there is no such implementation.

Author: jeanPerier

flang/lib/Optimizer/Builder/IntrinsicCall.cpp

@@ -684,6 +684,20 @@ static constexpr IntrinsicHandler handlers[]{
      /*isElemental=*/true},
 };
 
+template <std::size_t N>
+static constexpr bool isSorted(const IntrinsicHandler (&array)[N]) {
+  // Replace by std::sorted when C++20 is default (will be constexpr).
+  const IntrinsicHandler *lastSeen{nullptr};
+  bool isSorted{true};
+  for (const auto &x : array) {
+    if (lastSeen)
+      isSorted &= std::string_view{lastSeen->name} < std::string_view{x.name};
+    lastSeen = &x;
+  }
+  return isSorted;
+}
+static_assert(isSorted(handlers) && "map must be sorted");
+
 static const IntrinsicHandler *findIntrinsicHandler(llvm::StringRef name) {
   auto compare = [](const IntrinsicHandler &handler, llvm::StringRef name) {
     return name.compare(handler.name) > 0;