Per-column collation support

This adds collation support for columns and domains, a COLLATE clause
to override it per expression, and B-tree index support.

Peter Eisentraut
reviewed by Pavel Stehule, Itagaki Takahiro, Robert Haas, Noah Misch

src/backend/access/index/indexam.c

@@ -872,6 +872,8 @@ index_getprocinfo(Relation irel,
 				 procnum, attnum, RelationGetRelationName(irel));
 
 		fmgr_info_cxt(procId, locinfo, irel->rd_indexcxt);
+		fmgr_info_collation(irel->rd_index->indcollation.values[attnum-1],
+							locinfo);
 	}
 
 	return locinfo;