macros: The line number that `panic!` reports can be wrong in macro invocations

For [example](https://play.rust-lang.org/?gist=e3c24e819b9250e11399c19c682c8b2d&version=nightly&backtrace=0),
```rust
macro_rules! identity_macro { ($($t:tt)*) => { $($t)* } }

fn main() {
    identity_macro! { //< Today, the message says we panic here.
        panic!("")
    }
}
```