Debuginfo for `windows-msvc` treats `()` as `tuple$<>`

[wesleywiser]

@michaelwoerister found, debuginfo for the windows-msvc targets currently names the () type as tuple$<>. We should probably try to name the unit type () for consistency with Rust.

Relatedly, it may make sense to have natvis which sets a <DisplayString /> for ZSTs to just the type name.