Rename Ptr<T> to Nonnull<T*> (#832)

Note that ptr.h also includes an enable_if change, to help avoid https://bugs.llvm.org/show_bug.cgi?id=51881

executable_semantics/common/arena.h

@@ -16,10 +16,10 @@ class Arena {
  public:
   // Allocates an object in the arena, returning a pointer to it.
   template <typename T, typename... Args>
-  auto New(Args&&... args) -> Ptr<T> {
+  auto New(Args&&... args) -> Nonnull<T*> {
     auto smart_ptr =
         std::make_unique<ArenaEntryTyped<T>>(std::forward<Args>(args)...);
-    Ptr<T> ptr = smart_ptr->Instance();
+    Nonnull<T*> ptr = smart_ptr->Instance();
     arena.push_back(std::move(smart_ptr));
     return ptr;
   }
@@ -40,7 +40,7 @@ class Arena {
     explicit ArenaEntryTyped(Args&&... args)
         : instance(std::forward<Args>(args)...) {}
 
-    auto Instance() -> Ptr<T> { return Ptr<T>(&instance); }
+    auto Instance() -> Nonnull<T*> { return Nonnull<T*>(&instance); }
 
    private:
     T instance;

executable_semantics/common/ptr.h

@@ -5,11 +5,18 @@
 #ifndef EXECUTABLE_SEMANTICS_COMMON_PTR_H_
 #define EXECUTABLE_SEMANTICS_COMMON_PTR_H_
 
+#include <type_traits>
+
 namespace Carbon {
 
-// A non-nullable pointer. Written as `Ptr<T>` instead of `T*`.
-template <typename T>
-using Ptr = T* _Nonnull __attribute__((nonnull));
+// A non-nullable pointer. Written as `Nonnull<T*>` instead of `T*`.
+//
+// Note LLVM primarily enforces the attribute on function calls that can be
+// proven to be called with nullptr; in other places, this is essentially a
+// comment.
+template <typename T,
+          typename std::enable_if_t<std::is_pointer_v<T>>* = nullptr>
+using Nonnull = T _Nonnull __attribute__((nonnull));
 
 }  // namespace Carbon