Implement LWG 2946, 3075 and 3076. Reviewed as https://reviews.llvm.org/D48616

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@336132 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/strings/basic.string/string.modifiers/string_append/string.pass.cpp

@@ -77,4 +77,13 @@ int main()
          S("1234567890123456789012345678901234567890"));
     }
 #endif
+
+#if TEST_STD_VER > 3
+    {   // LWG 2946
+    std::string s;
+    s.append({"abc", 1});
+    assert(s.size() == 1);
+    assert(s == "a");
+    }
+#endif
 }