replace empty destructors with NonCopyable

[thestinger]

The NonCopyable unit struct is now zero-size via #[unsafe_no_drop_flag], so it's the cleanest way to do this.

This could potentially be a lint warning, with a suggestion.

[bstrie]

A lint for empty drops is probably unnecessary, I wouldn't imagine that would be a common idiom once NonCopyable is well-known.

[brson]

I only see one of these, in libsyntax.