libstdc++: Fix std::string error in Debug Mode

This fixes a test failure with -std=gnu++98 -D_GLIBCXX_DEBUG:

FAIL: 21_strings/basic_string/modifiers/insert/char/1.cc (test for excess errors)

	* include/debug/string (__gnu_debug::basic_string::insert): Fix for
	C++98 where the member function of the base class returns void.

libstdc++-v3/ChangeLog

 2020-02-27  Jonathan Wakely  <jwakely@redhat.com>
 
+	* include/debug/string (__gnu_debug::basic_string::insert): Fix for
+	C++98 where the member function of the base class returns void.
+
 	* testsuite/util/testsuite_iterators.h (forward_iterator_wrapper): Add
 	equality comparisons that support value-initialized iterators.
 

libstdc++-v3/include/debug/string

@@ -635,7 +635,7 @@ namespace __gnu_debug
 	  __glibcxx_check_insert_range(__p, __first, __last, __dist);
 
 	  typename _Base::iterator __res;
-#if _GLIBCXX_USE_CXX11_ABI
+#if _GLIBCXX_USE_CXX11_ABI && __cplusplus >= 201103
 	  if (__dist.second >= __dp_sign)
 	    __res = _Base::insert(__p.base(), __gnu_debug::__unsafe(__first),
 				  __gnu_debug::__unsafe(__last));