There was a FIXME noting that ty::enum_variants and typeck::check::
check_enum_variants both call eval_const_expr. I tried refactoring the
code so that check_enum_variants does all the work and enum_variants
just looks up cached results, but this turned out not to be easy because
several ty functions call enum_variants and it might get invoked on an
enum before that enum item has been typechecked. Instead, I just made
check_enum_variants update the enum_var_cache so that enum_variants
will never call eval_const_expr twice on the same const.