Add result sanity check to `is_to_be_inferred`.

Add result sanity check to `is_to_be_inferred`.

This version of `is_to_be_inferred` double-checks the result from
`inferred_map` by querying the `named_region_map` and `ast_map` and
then asserts that the `inferred_map` state is consistent with its own
findings.  (See issue 13261 for further discussion of the approaches).