Standardize on log-based undo #23357
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Drop the snapshot scheme in VarState. The previous scheme did leak instantiations from failed subtype comparisons, since a subtype comparison might have several subCaptures tests. If a first subCaptures test succeeds then anything it instantiated is made permanent, even if a subsequent test and the whole subType comparison fails.
The change needed updates in several neg test files where we find that fewer things are instantiated in error messages.