Normative: Apply chain rule to IsFunctionDefinition UnaryExpression: …

…UpdateExpression (#706)

Since a *FunctionExpression* is a *PrimaryExpression*, I believe
returning `false` in `IsFunctionDefinition` for every production of
*UnaryExpression* is incorrect. It's also not consistent with the rules for
every other expression.

Otherwise I think `IsFunctionDefinition` would return `false` when
evaluating initializers (e.g. `function foo(bar=function(){}) {}`),
after "unrolling" all production rules.

spec.html

@@ -12254,7 +12254,6 @@ <h1>Static Semantics: IsFunctionDefinition</h1>
       <emu-see-also-para op="IsFunctionDefinition"></emu-see-also-para>
       <emu-grammar>
         UnaryExpression :
-          UpdateExpression
           `delete` UnaryExpression
           `void` UnaryExpression
           `typeof` UnaryExpression