[10.x] Convert eloquent builder to base builder in whereExists

[gdebrauwer]

The whereExists method accepts an Eloquent builder (see docblock on whereExists-methods) but the addWhereExistsQuery method used by whereExists only accepts a base builder. To fix that I added code that converts Eloquent builder to base builder before calling the addWhereExistsQuery method.

(The other solution would be changing the parameter type on the addWhereExistsQuery method, but I thought converting the type in the whereExists method would probably be the preferred way of handling this)