cql3/prepare_expr: fix partially preparing function arguments

[cvybhu]

Before choosing a function, we prepare the arguments that can be prepared without a receiver. Preparing an argument makes its type known, which allows to choose the best overload among many possible functions.

The function that prepared the argument passes the unprepared argument by mistake. Let's fix it so that it actually uses the prepared argument.

[cvybhu]

I didn't add any unit tests, because I couldn't find any cases that would be affected by this bug.

Why aren't there any?
I originally added the partial preparing because test_assignment didn't work well on unprepared arguments. In particular I remember that trying to run test_assignment on an unprepared unresolved_identifier caused an error, so all columns had to be prepared before choosing a function.

Now I see that test_assignment prepares unprepared expressions, so this issue doesn't appear.
I'm not sure how valid this is. prepare_expression is supposed to throw an error when the receiver type doesn't match, but test_assignment shouldn't throw any exceptions on type mismatch, it should return a value.

We can consider some changes in this area, but for now let's fix the buggy code, so that it does what it was supposed to.

[scylladb-promoter]

CI state SUCCESS - https://jenkins.scylladb.com/job/scylla-master/job/scylla-ci/2625/

[nyh]

Looks good to me. I have to admit I didn't understand why it's impossible to write a unit test that would have caught this bug, but I agree it looks like a straighforward typo that should be fixed so I don't need a test to be convinced.
IIUC, this is a change to very new code (8d3d8ee) so it won't need a backport so I'll avoid my usual request to open issues when fixing bugs, but in general - we should.