Function walk_chain_collapsed

pub fn walk_chain_collapsed(span: Span, to: Span) -> Span

In order to have good line stepping behavior in debugger, for the given span we return its outermost macro call site that still has a #[collapse_debuginfo(yes)] property on it. We also stop walking call sites at the function body level because no line stepping can occur at the level above that. The returned span can then be used in emitted debuginfo.