Fix parsing of an "async of" edge case in for loop

[adams85]

The following edge case of for loops should result in syntax error but is accepted currently:

async () => { for (async
of []); }

This PR makes an attempt at fixing this issue.

Also, suggests a minor improvement to parsing of for statements:

As far as I can tell based on the spec, we can "take a shortcut" in the case of for-await-of loops. Once we read "for await", the initializer of the for statement is no longer ambiguous (since we know that it can't be a for(;;) loop), that is, the initializer part will be a LeftHandSideExpression (unless it's a variable declaration). So, we don't need to parse a full expression using parseExpression, parseExprSubscripts is enough - if my understanding of acorn's inner workings is correct.

Also V8 seems to take this shortcut: just check the error reported for e.g. async() => { for await (x = 1 of []); }

[marijnh]

Thanks for finding that!