Keep non-brace macro invocations in trailing expr position as Expr::Macro

[dtolnay]

Rust does not allow a stmt macro to be called with this syntax.

macro_rules! m {
    () => {
        struct S;
    }
}

fn main() {
    m!()
}

But if the call is written as m!(); or m! {}, we'll continue to parse it as Stmt::Macro.