Fix __invoke_r<void> to be valid in C++11

	* include/bits/invoke.h [__cplusplus < 201703L] (__invoke_r<void>):
	Use _GLIBCXX14_CONSTEXPR because void functions cannot be constexpr
	in C++11.

From-SVN: r271317

libstdc++-v3/ChangeLog

 2019-05-17  Jonathan Wakely  <jwakely@redhat.com>
 
+	* include/bits/invoke.h [__cplusplus < 201703L] (__invoke_r<void>):
+	Use _GLIBCXX14_CONSTEXPR because void functions cannot be constexpr
+	in C++11.
+
 	* doc/xml/manual/status_cxx2020.xml: Update P0608R3, P0777R1, and
 	P1165R1 entries.
 	* doc/html/*: Regenerate.

libstdc++-v3/include/bits/invoke.h

@@ -144,7 +144,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
   // INVOKE<R> when R is cv void
   template<typename _Res, typename _Callable, typename... _Args>
-    constexpr __can_invoke_as_void<_Res, _Callable, _Args...>
+    _GLIBCXX14_CONSTEXPR __can_invoke_as_void<_Res, _Callable, _Args...>
     __invoke_r(_Callable&& __fn, _Args&&... __args)
     {
       using __result = __invoke_result<_Callable, _Args...>;