Only qualify function as constexpr for C++14 and later

This helper function is not a valid constexpr function in C++11, so
should only be marked constexpr for C++14 and later.

	* include/debug/helper_functions.h (__valid_range): Change
	_GLIBCXX_CONSTEXPR to _GLIBCXX14_CONSTEXPR.

From-SVN: r277338

libstdc++-v3/ChangeLog

 2019-10-23  Jonathan Wakely  <jwakely@redhat.com>
 
+	* include/debug/helper_functions.h (__valid_range): Change
+	_GLIBCXX_CONSTEXPR to _GLIBCXX14_CONSTEXPR.
+
 	* include/ext/throw_allocator.h (throw_allocator_base): Qualify
 	size_t and ptrdiff_t.
 

libstdc++-v3/include/debug/helper_functions.h

@@ -221,7 +221,7 @@ namespace __gnu_debug
 #endif
 
   template<typename _InputIterator>
-    _GLIBCXX_CONSTEXPR
+    _GLIBCXX14_CONSTEXPR
     inline bool
     __valid_range(_InputIterator __first, _InputIterator __last)
     {