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_insert/size_string.pass.cpp

@@ -218,4 +218,13 @@ int main()
     test(S("abcdefghijklmnopqrst"), 21, S("12345678901234567890"), S("can't happen"));
     }
 #endif
+
+#if TEST_STD_VER > 3
+    {   // LWG 2946
+    std::string s;
+    s.insert(0, {"abc", 1});
+    assert(s.size() == 1);
+    assert(s == "a");
+    }
+#endif
 }