Propagate oneofs_nonexhaustive value to descendant contexts #2105

Job	Run time
linux nightly (all features)	6m 43s
Check sync-readme	48s
linux beta (default features)	7m 13s
linux stable (default features)	7m 42s
linux stable (with-bytes)	6m 22s
mega-linter	2m 18s
Miri test	46s
rustfmt check	8m 15s
CI self-check	13s
windows stable (default features)	4m 53s
	45m 13s

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