Make make_folded_const return overflow as [[nodiscard]] bool

Previously make_folded_const returned void and reported overflow via a
bool& out parameter. Per review, callers could miss handling overflow
because the parameter is easy to overlook. Make the function return
the overflow flag directly, marked [[nodiscard]] so the compiler
enforces that callers consider it.

Callers that recurse accumulate via |=; consume sites (Overflows,
Overflow predicates, evaluate_predicate's gating logic) read the
return value directly. The constant_fold_bin_op helpers still take
bool& since they already return the computed value.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: abadams