Store metadata about unresolved materializations in a separate data
structure. This is in preparation of the One-Shot Dialect Conversion
refactoring, which no longer maintains a stack of `IRRewrite` objects.
Therefore, metadata about unresolved materializations can no longer be
retrieved from `UnresolvedMaterializationRewrite` objects.
This commit also removes a pointer indirection and may slightly improve
the performance of the existing driver.