[m-rev.] for post-commit review: stricter checking of tabling pragmas vs the grade

[Zoltan Somogyi]

There is a minor independent change to mercury_grade.h as well.
For review by anyone.

Zoltan.
-------------- next part --------------
A non-text attachment was scrubbed...
Name: Log.tem
Type: application/octet-stream
Size: 1880 bytes
Desc: not available
URL: <http://lists.mercurylang.org/archives/reviews/attachments/20210614/302911d2/attachment-0003.obj>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: DIFF.tem
Type: application/octet-stream
Size: 100522 bytes
Desc: not available
URL: <http://lists.mercurylang.org/archives/reviews/attachments/20210614/302911d2/attachment-0004.obj>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: DIFF.tngc
Type: application/octet-stream
Size: 2300 bytes
Desc: not available
URL: <http://lists.mercurylang.org/archives/reviews/attachments/20210614/302911d2/attachment-0005.obj>