cgen: fix option var nested unwrapping from sumtype (fix #23478) (#23… #424

Job	Run time
tests-sanitize-address-clang	2h 34m 21s
tests-sanitize-address-msvc	3m 59s
tests-sanitize-address-gcc	2h 17m 29s
tests-sanitize-memory-clang	2h 48m 6s
tests-sanitize-undefined-clang	44m 20s
tests-sanitize-undefined-gcc	59m 44s
	9h 27m 59s

Provide feedback