To ensure that we do not accept code that would require an existential call argument
to be evaluated prior to the call, don't open existential call
arguments if the generic parameter that would capture the opened
existential type is used in any prior parameter.
Note that code generation currently moves the opening of the
existential call argument outside of the call, which is a compiler bug.
This change ensures that we don't accept code would *require* this
out-of-order evaluation and adds a test so we know when we fix this.
