fix CI, sem whole when stmts as generic stmt

[metagn]

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}.

[github-actions]

Thanks for your hard work on this PR!
The lines below are statistics of the Nim compiler built from 79a65da

Hint: mm: orc; opt: speed; options: -d:release
174174 lines; 8.338s; 655.215MiB peakmem