Fix pretty printing of empty bound lists in where-clause

[dtolnay]

Repro:

macro_rules! assert_item_stringify {
    ($item:item $expected:literal) => {
        assert_eq!(stringify!($item), $expected);
    };
}

fn main() {
    assert_item_stringify! {
        fn f<'a, T>() where 'a:, T: {}
        "fn f<'a, T>() where 'a:, T: {}"
    }
}

Previously this assertion would fail because rustc renders the where-clause as where 'a, T which is invalid syntax.

This PR makes the above assertion pass.

This bug also affects -Zunpretty=expanded. The intention is for that to emit syntactically valid code, but the buggy output is not valid Rust syntax.

$ rustc <(echo "fn f<'a, T>() where 'a:, T: {}") -Zunpretty=expanded
#![feature(prelude_import)]
#![no_std]
#[prelude_import]
use ::std::prelude::rust_2015::*;
#[macro_use]
extern crate std;
fn f<'a, T>() where 'a, T {}

$ rustc <(echo "fn f<'a, T>() where 'a:, T: {}") -Zunpretty=expanded | rustc -
error: expected `:`, found `,`
 --> <anon>:7:23
  |
7 | fn f<'a, T>() where 'a, T {}
  |                       ^ expected `:`

[rust-highfive]

r? @lcnr

(rust-highfive has picked a reviewer for you, use r? to override)

[lcnr]

@bors r+ rollup

[bors]

📌 Commit fe9635f has been approved by lcnr