update diagnostic message

Author: a-tarasyuk

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -1031,7 +1031,7 @@ def err_mainlike_template_decl : Error<"%0 cannot be a template">;
 def err_main_returns_nonint : Error<"'main' must return 'int'">;
 def ext_main_returns_nonint : ExtWarn<"return type of 'main' is not 'int'">,
     InGroup<MainReturnType>;
-def ext_main_no_return : Extension<"non-void 'main' function does not return a value">,
+def ext_main_no_return : Extension<"implicit '0' return value from 'main' is a C99 extension">,
     InGroup<MainReturnType>;
 def note_main_change_return_type : Note<"change return type to 'int'">;
 def err_main_surplus_args : Error<"too many parameters (%0) for 'main': "