Comment 8 for bug 1791425

Another reason why we badly need to finish PR81601.
The constexpr evaluator really can't cope well with these BIT_FIELD_REFs, they read multiple fields at once, or could also read some uninitialized bits in between and mask them off, etc.
So at minimum we'd need to disable these optimizations while folding constexpr declared functions and if the fields don't have INTEGRAL_TYPE or the bit field reference covers more than one field.