Copy the relation name for error reporting in WCOs

Copy the relation name for error reporting in WCOs

In get_row_security_policies(), we need to make a copy of the relation
name when building the WithCheckOptions structure, since
RelationGetRelationName just returns a pointer into the local Relation
structure.  The relation name in the WCO structure is only used for
error reporting.

Pointed out by Robert and Christian Ullrich, who noted that the
buildfarm members with -DCLOBBER_CACHE_ALWAYS were failing.