Derive `Arbitrary` for enums with a single variant #8925

Job	Run time
regression (macos-13)	1h 1m 57s
write-json-symtab-regression	17m 11s
benchcomp-tests	4m 24s
regression (ubuntu-20.04)	28m 30s
documentation	2m 16s
regression (ubuntu-22.04)	29m 2s
perf	30m 45s
regression (macos-14)	27m 57s
	3h 22m 2s

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