Add mstd_new for mstd::launder

Increasingly clever compilers can hit funny aliasing problems with
object stores like mbed::Callback. Add access to the C++17 launder
facility.

platform/cxxsupport/mstd_new

@@ -0,0 +1,56 @@
+/* mbed Microcontroller Library
+ * Copyright (c) 2019 ARM Limited
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+#ifndef MSTD_NEW_
+#define MSTD_NEW_
+
+/* <mstd_new>
+ *
+ * - includes toolchain's <new>
+ * - For all toolchains, C++17 backports:
+ *   - mstd::launder
+ */
+
+#include <new>
+#if __cpp_lib_launder < 201606
+#include <type_traits>
+#endif
+
+namespace mstd
+{
+using std::nothrow_t;
+using std::nothrow;
+using std::new_handler;
+using std::set_new_handler;
+
+#if __cpp_lib_launder >= 201606
+using std::launder;
+#else
+template <typename T>
+constexpr T *launder(T *p) noexcept
+{
+    static_assert(!std::is_function<T>::value && !std::is_void<T>::value, "Can only launder complete object types");
+#if defined __clang__ || __GNUC__ >= 9
+    return __builtin_launder(p);
+#else
+    return p;
+#endif
+}
+#endif
+
+} // namespace mstd
+
+#endif // MSTD_NEW_