Follow-up to d6fbffa23c84e622735b3e880fd800985c1c0072 . This commit updates all call sites and removes the argument from the function.
Allows MCExprs as passed values to PALMetadata. Also adds related `DelayedMCExpr` classes which serve as a pseudo-fixup to resolve MCExprs as late as possible (i.e., right before emit through string or blob, where they should be resolvable).