Change Declaration passing to use Ptr (#758)

executable_semantics/ast/BUILD

@@ -19,6 +19,7 @@ cc_library(
         ":pattern",
         ":struct_definition",
         "//common:ostream",
+        "//executable_semantics/common:ptr",
         "@llvm-project//llvm:Support",
     ],
 )

executable_semantics/ast/abstract_syntax_tree.h

@@ -8,9 +8,10 @@
 #include <variant>
 
 #include "executable_semantics/ast/declaration.h"
+#include "executable_semantics/common/ptr.h"
 
 namespace Carbon {
-using AST = std::list<const Carbon::Declaration*>;
+using AST = std::list<Ptr<const Declaration>>;
 }
 
 #endif  // EXECUTABLE_SEMANTICS_AST_ABSTRACT_SYNTAX_TREE_H_