Recursively respan tokens interpolated from a macro_rules metavariable

[dtolnay]

Repro:

macro_rules! repro {
    ($span:expr=> $expr:expr) => {
        quote_spanned!($span=> $expr)
    };
}

Previously an invocation like repro!(span=> a + (b)) would set the specified span on the top-level tokens only, i.e. a and + and the paren token, but not on the b token.