Editorial: Allow ACs in Async{Function,Block}Start (#3452)

There is no use of Abstract Closures as async function bodies in ecma262 itself.
This PR is a prereq to allow the JSPI specification to pass in an AC.

spec.html

@@ -49462,7 +49462,7 @@ <h1>Async Functions Abstract Operations</h1>
         <h1>
           AsyncFunctionStart (
             _promiseCapability_: a PromiseCapability Record,
-            _asyncFunctionBody_: a |FunctionBody| Parse Node or an |ExpressionBody| Parse Node,
+            _asyncFunctionBody_: a |FunctionBody| Parse Node, an |ExpressionBody| Parse Node, or an Abstract Closure with no parameters,
           ): ~unused~
         </h1>
         <dl class="header">
@@ -49480,7 +49480,7 @@ <h1>
         <h1>
           AsyncBlockStart (
             _promiseCapability_: a PromiseCapability Record,
-            _asyncBody_: a Parse Node,
+            _asyncBody_: a Parse Node or an Abstract Closure with no parameters,
             _asyncContext_: an execution context,
           ): ~unused~
         </h1>
@@ -49490,7 +49490,11 @@ <h1>
           1. Let _runningContext_ be the running execution context.
           1. Let _closure_ be a new Abstract Closure with no parameters that captures _promiseCapability_ and _asyncBody_ and performs the following steps when called:
             1. Let _acAsyncContext_ be the running execution context.
-            1. Let _result_ be Completion(Evaluation of _asyncBody_).
+            1. If _asyncBody_ is a Parse Node, then
+              1. Let _result_ be Completion(Evaluation of _asyncBody_).
+            1. Else,
+              1. Assert: _asyncBody_ is an Abstract Closure with no parameters.
+              1. Let _result_ be _asyncBody_().
             1. Assert: If we return here, the async function either threw an exception or performed an implicit or explicit return; all awaiting is done.
             1. Remove _acAsyncContext_ from the execution context stack and restore the execution context that is at the top of the execution context stack as the running execution context.
             1. If _result_ is a normal completion, then