[php-langspec][Issue #235] - yield and array initializer precedence not specified correctly

You can view the Issue on github:
https://github.com/php/php-langspec/issues/235

Opened By: nikic
Issue Description:
Split off from #208:

Currently [array-element-initializer](https://github.com/php/php-langspec/blob/master/spec/10-expressions.md#grammar-array-element-initializer) specifies that the key and value are both ordinary expressions. As specified, this is ambiguous with [yield-expression](https://github.com/php/php-langspec/blob/master/spec/10-expressions.md#grammar-yield-expression).

PHP resolves this as follows:

```php
[yield "foo" => "bar"]
// is
[(yield "foo" => "bar")]
// rather than
[(yield "foo") => "bar"]
```

However, I'm not sure how this can be specified in grammar form.