Rollup merge of #60590 - petertodd:2018-test-union-nonzero, r=nikomatsakis,Centril

Rollup merge of #60590 - petertodd:2018-test-union-nonzero, r=nikomatsakis,Centril

Test interaction of unions with non-zero/niche-filling optimization

Notably this nails down part of the behavior that MaybeUninit assumes, e.g. that a Option<MaybeUninit<&u8>> does not take advantage of non-zero optimization, and thus is a safe construct.

It also verifies the status quo: that even unions that could theoretically take advantage of niches don't. (relevant: https://github.com/rust-lang/rust/issues/36394)