Make Directive | Statement comments meaningful (estree#173)

* Make Directive | Statement comments meaningful

Because a Directive is an ExpressionStatement and an ExpressionStatement is a Statement, the use of Directive | Statement option was redundant. This change attempts to capture the intent of the Directive | Statement, by making it clear that while Directive is structurally a subtype of ExpressionStatement, the directive type is not a valid addition to any ExpressionStatement, just ones that are children of a Function or Program.

* Make Directive hold only Literal expressions.

At the suggestion of @RReverser, express the further refinement that the subexpression of a Directive is always a Literal.

es5.md

@@ -177,7 +177,9 @@ An expression statement, i.e., a statement consisting of a single expression.
 ## Directive
 
 ```js
-interface Directive <: ExpressionStatement {
+interface Directive <: Node {
+    type: "ExpressionStatement";
+    expression: Literal;
     directive: string;
 }
 ```