Add Ptr and RawNew for migration (#751)

Co-authored-by: Geoff Romer <gromer@google.com>

executable_semantics/common/ptr.h

@@ -0,0 +1,43 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#ifndef EXECUTABLE_SEMANTICS_COMMON_PTR_H_
+#define EXECUTABLE_SEMANTICS_COMMON_PTR_H_
+
+#include <memory>
+#include <vector>
+
+#include "common/check.h"
+
+namespace Carbon {
+
+// A non-nullable pointer. Written as `Ptr<T>` instead of `T*`.
+template <typename T>
+class Ptr {
+ public:
+  explicit Ptr(T* ptr) : ptr(ptr) { CHECK(ptr != nullptr); }
+
+  template <typename OtherT,
+            std::enable_if_t<std::is_convertible_v<OtherT*, T*>>* = nullptr>
+  Ptr(Ptr<OtherT> other) {
+    return Ptr<OtherT>(other.ptr);
+  }
+
+  Ptr(std::nullptr_t) = delete;
+
+  Ptr(const Ptr& other) = default;
+  Ptr& operator=(const Ptr& rhs) = default;
+
+  auto operator*() const -> T& { return *ptr; }
+  auto operator->() const -> T* { return ptr; }
+
+  T* Get() const { return ptr; }
+
+ private:
+  T* ptr;
+};
+
+}  // namespace Carbon
+
+#endif  // EXECUTABLE_SEMANTICS_COMMON_PTR_H_