fixes#8758, fixes#10828, fixes#12172, fixes#21610, fixes#23803,
fixes#24633, fixes#24634, succeeds #24085
We simply track the symbol ID of every traversed `var`/`let` definition
in `vmgen`, then these symbols are always considered evaluable in the
current `vmgen` context. The set of symbols is reset before every
generation, but both tests worked properly without doing this including
the nested `const`, so maybe it's already done in some way I'm not
seeing.
(cherry picked from commit a5cc33c1d3)
