Convert Statement to use Ptr (#788)

Note this makes a few cases where the Statement was optional explicit (Block, If, Sequence). I do add a few CHECKs around where statements were optional and assumed but unverified.

I switch TypeCheckStmt to not take an optional Statement because I think it makes the call sites clearer in behavior. It's also a smaller change than the converse, because taking an optional Statement means the returned statement would also need to be optional. Arguably a wrapper for optional statements could be added, but this still seems cleaner to me, and there aren't that many cases of an optional statement.

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

executable_semantics/ast/function_definition.h

@@ -26,7 +26,8 @@ struct FunctionDefinition {
                      std::vector<GenericBinding> deduced_params,
                      Ptr<const TuplePattern> param_pattern,
                      Ptr<const Pattern> return_type,
-                     bool is_omitted_return_type, const Statement* body)
+                     bool is_omitted_return_type,
+                     std::optional<Ptr<const Statement>> body)
       : source_location(source_location),
         name(std::move(name)),
         deduced_parameters(deduced_params),
@@ -45,7 +46,7 @@ struct FunctionDefinition {
   Ptr<const TuplePattern> param_pattern;
   Ptr<const Pattern> return_type;
   bool is_omitted_return_type;
-  const Statement* body;
+  std::optional<Ptr<const Statement>> body;
 };
 
 }  // namespace Carbon