fix(binapigen): Fix name conflict for union field constructors

PROBLEM

Issue discovered in flow_types.api on master (20.09-rc0), where
generator encountered inconsistent naming for union type Flow,
causing name conflicts with their constructors.

SOLUTION

Previous cases of union types (address, punt) both contain suffix
"Union", thus generator now adds "Union" suffix it is not defined.
This way we won't break previously generated code for users.

Change-Id: Iffadc167774d66d8416fe36485782bb68ca2a70d
Signed-off-by: Ondrej Fabry <ofabry@cisco.com>