Move pg_upgrade_support global variables to their own include file

contrib/pg_upgrade_support/pg_upgrade_support.c

src/backend/catalog/heap.c

src/backend/catalog/index.c

src/backend/catalog/pg_enum.c

src/backend/catalog/pg_type.c

src/backend/catalog/toasting.c

src/backend/commands/typecmds.c

src/backend/commands/user.c

src/include/catalog/binary_upgrade.h