feature gate trace_macros! macro

[brson]

Don't know what this does. Nominating.

[huonw]

trace_macros just prints the arguments a macro is called with, for every expansion. e.g.

#[feature(macro_rules)];

macro_rules! macro {
    () => { println!("hello"); };
    ($y:tt $($x:tt)*) => {
        macro!($($x)*)
    }
}

fn main() {
    trace_macros!(true);
    macro!(1 2 3 4)
}

$ rustc trace_macros.rs
macro! { 1 2 3 4 }
macro! { 2 3 4 }
macro! { 3 4 }
macro! { 4 }
macro! {  }
println! { "hello" }

(This isn't implying that it shouldn't be feature gated though.)

[pnkfelix]

Accepted for 1.0, P-backcompat-lang. (same reasoning as #11630)