[flang] Allow use of (x?"a":"b") as a message argument

Original-commit: flang-compiler/f18@c0d4e55455
Reviewed-on: https://github.com/flang-compiler/f18/pull/451

flang/lib/parser/message.h

@@ -101,6 +101,8 @@ private:
     static_assert(!std::is_class_v<std::decay_t<A>>);
     return std::move(x);
   }
+  const char *Convert(const char *s) { return s; }
+  const char *Convert(char *s) { return s; }
   const char *Convert(const std::string &);
   const char *Convert(std::string &);
   const char *Convert(std::string &&);