EExpArgGroup should tag child values with the corresponding parameter

[zslayton]

When a macro emits an argument value that was originally passed in as part of an argument group, the value isn't tagged with the group's corresponding parameter.

We should only show the encoding of 1 2 3 inside the e-expression argument group, but we show their encoding again when they're emitted by values because they aren't tagged with x.

[zslayton]

On closer inspection, I believe that the problem may be that (:: ...) is tagging its child expressions with x, but then (:values ...) is setting their variable back to None because (:values ...) itself doesn't have a variable.

It should be possible for tags to be overwritten with a new variable, but not unset altogether.