Fix creation of overlapping GROUPING and regular FOREIGN KEYs

And reject inline (CREATE, ALTER TABLE) group index definitions as the underlying code doesn't expect them.

This is mostly solved by copying more information to the "stub" table used by TableDDL and AlterTableDDL. Instead of trying to figure out exactly what needs to be present just copy the table (entire groups, actually) wholesale. Simplifies the code a bit and seems less likely to miss something.
A group index definition was contained in an ALTER ADD INDEX statement.

Would "ALTER ADD INDEX does not support group indexes" be clearer?