"could not find definition corresponding to production X" warning in proposals

[bakkot]

We give a warning when encountering an SDO defined over a production which isn't defined in the current document. In proposals, which might be adding new SDOs for upstream productions, that warning isn't useful. We should suppress it in proposals, possibly by making it conditional on --lint-spec or by introducing a new flag to indicate whether the document is intended to be a complete specification.