Fix PR#25973 : 'basic_string::assign(InputIt, InputIt) doesn't provide the strong exception safety guarantee'. This turned out to be a pervasive problem in <string>, which required a fair amount of rework. Add in an optimization for when iterators provide noexcept increment/comparison/assignment/dereference (which covers many of the iterators in libc++). Reviewed as http://reviews.llvm.org/D15862

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@257682 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/strings/basic.string/string.modifiers/string_insert/iter_char.pass.cpp

@@ -56,7 +56,7 @@ int main()
     test(s, s.begin()+5, 'B', S("a567AB1432dcb"));
     test(s, s.begin()+6, 'C', S("a567ABC1432dcb"));
     }
-#if __cplusplus >= 201103L
+#if TEST_STD_VER >= 11
     {
     typedef std::basic_string<char, std::char_traits<char>, min_allocator<char>> S;
     S s;