Rename Aggreg to Aggref.

src/backend/executor/execQual.c

src/backend/executor/nodeAgg.c

src/backend/nodes/copyfuncs.c

src/backend/nodes/outfuncs.c

src/backend/nodes/readfuncs.c

src/backend/optimizer/plan/setrefs.c

src/backend/optimizer/plan/subselect.c

src/backend/optimizer/util/clauses.c

src/backend/optimizer/util/tlist.c

src/backend/optimizer/util/var.c

src/backend/parser/gram.c

src/backend/parser/parse_agg.c

src/backend/parser/parse_expr.c

src/backend/parser/parse_target.c

src/backend/rewrite/locks.c

src/backend/rewrite/rewriteHandler.c

src/backend/rewrite/rewriteManip.c

src/backend/utils/adt/ruleutils.c

src/include/nodes/nodes.h

src/include/nodes/primnodes.h