Problem:
After it was made possible to merge derived tables into the outer
query block, it was no longer possible to use a documented workaround
for how to avoid error ER_UPDATE_TABLE_USED when referencing the same
table in a subquery as was used as target for an UPDATE or DELETE
statement.

The solution is to put tighter control over which derived tables that
can be merged. A boolean property is introduced in class SELECT_LEX,
controlling which derived tables underneath it that can be merged:
The important derived tables to control are those immediately
underlying some subqueries of an UPDATE or DELETE statement.

Notice that this property affects unnamed derived tables only;
view references are not affected.