This avoids another unserializable field. Move the DbgInfoAvailable
field into the AsmPrinter, which is only really a cache/convenience
bit for checking a direct IR module metadata check.
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).