Array members are allow to have attributes such as `#[cfg]`.
This is a bit tricky as we don't know if the first expression is an
initializer or a member until we encounter a `;`. This reuses a trick
from `stmt` where we remember if we saw an attribute and then raise an
error if the first expression ends up being an initializer.
This isn't perfect as the error isn't correctly located on the attribute
or initializer; it ends up immediately after the `;`.
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