libstdc++: Fix failing test in debug mode

This fixes a failure due to a (correct) warning seen when testing with
-D_GLIBCXX_USE_CXX11_ABI=0 -D_GLIBCXX_ASSERTIONS:

include/bits/char_traits.h:365: warning: 'void* __builtin_memcpy(void*, const void*, long unsigned int)'
specified bound 18446744073709551615 exceeds maximum object size 9223372036854775807 [-Wstringop-overflow=]

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

	* testsuite/21_strings/basic_string/cons/char/1.cc: Disable
	-Wstringop-overflow warnings.

libstdc++-v3/ChangeLog

+2020-02-28  Jonathan Wakely  <jwakely@redhat.com>
+
+	* testsuite/21_strings/basic_string/cons/char/1.cc: Disable
+	-Wstringop-overflow warnings.
+
 2020-02-27  Jonathan Wakely  <jwakely@redhat.com>
 
 	* testsuite/lib/libstdc++.exp (v3_target_compile): Add

libstdc++-v3/testsuite/21_strings/basic_string/cons/char/1.cc

@@ -19,6 +19,8 @@
 
 // 21.3.1 basic_string constructors.
 
+// { dg-options "-Wno-stringop-overflow" }
+
 #include <new>
 #include <stdexcept>
 #include <testsuite_hooks.h>