5326: infer: Add type inference support for Union types r=flodiebold a=otavio
This adds the type inference to Union types and add a small test case
for it, ensuring it keeps working in future.
Fixes: #5277
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
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Co-authored-by: Otavio Salvador <otavio@ossystems.com.br>