[Concepts] Transform constraints of non-template functions to ConstantEvaluated

We would previously try to evaluate atomic constraints of non-template functions as-is,
and since they are now unevaluated at first, this would cause incorrect evaluation (bugs #44657, #44656).

Substitute into atomic constraints of non-template functions as we would atomic constraints
of template functions, in order to rebuild the expressions in a constant-evaluated context.