checker: warn when accessing union fields outside unsafe
#7869
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Rust makes reading (and sometimes writing) union fields require
unsafe. This pull does so for both.https://doc.rust-lang.org/reference/items/unions.html#reading-and-writing-union-fields
Reading union fields is memory-unsafe if there is a type that is a pointer/reference field somewhere inside the union, and it can cause weird values if not used carefully - e.g. an enum field can get a value outside its members.