Allow reassignment of detached criteria variables

If you assign a Where query to an untyped variable like so:

def authors = Author.where { age > avg(age) }

then the authors variable is given an explicit type of DetachedCriteria by the underlying AST transform. This results in class cast errors if you do things like:

authors = "Some text"

This is very strange for users since the code is perfectly valid Groovy, it's just the AST transformation is having unintended side-effects.

See GRAILS-8355 for background and sample app.