Fix assignment to array of domain over composite, redux.

 Fix assignment to array of domain over composite, redux.
 
 Commit 3e310d837 taught isAssignmentIndirectionExpr() to look through
 CoerceToDomain nodes.  That's not sufficient, because since commit
 04fe805a1 it's been possible for the planner to simplify
 CoerceToDomain to RelabelType when the domain has no constraints
 to enforce.  So we need to look through RelabelType too.
 
 Per bug #17897 from Alexander Lakhin.  Although 3e310d837 was
 back-patched to v11, it seems sufficient to apply this change
 to v12 and later, since 04fe805a1 came in in v12.
 
 Dmitry Dolgov
 
 Discussion: https://postgr.es/m/17897-4216c546c3874044@postgresql.org