fixes CI, refs #24066, refs #24065
The combination of #24065 and #24066 caused a CI failure where a `when`
branch that was never compiled gave an undeclared identifier error. This
is because the `when` branch was being semchecked with `semGenericStmt`
without `withinMixin`, which is the flag `semgnrc` normally gives to
`when` branch bodies. To fix this, just pass the whole `when` stmt to
`semGenericStmt` rather than the individual blocks.
The alternative would be to just replace the calls to `semGenericStmt`
with a new proc that does the same thing, just with the flags
`{withinMixin}`.
79a65da22a