add warning for user-defined literal

Author: usx95

clang/test/SemaCXX/attr-lifetimebound.cpp

@@ -85,7 +85,7 @@ namespace std {
     char &operator[](size_t) const [[clang::lifetimebound]];
   };
   using string =  basic_string<char>;
-  string operator""s(const char *, size_t); // expected-warning {{}}
+  string operator""s(const char *, size_t); // expected-warning {{user-defined literal suffixes not starting with '_' are reserved}}
 
   template<typename T>
   struct basic_string_view {
@@ -94,7 +94,7 @@ namespace std {
     basic_string_view(const string &s [[clang::lifetimebound]]);
   };
   using string_view = basic_string_view<char>;
-  string_view operator""sv(const char *, size_t); // expected-warning {{}}
+  string_view operator""sv(const char *, size_t); // expected-warning {{user-defined literal suffixes not starting with '_' are reserved}}
 
   struct vector {
     int *data();