Rustfmt creates invalid syntax on extern block containing macro call

[dtolnay]

macro_rules! x {
    ($tt:tt) => {};
}

extern "C" {
    x!(-);
    x!(x);
}

Rustfmt loses the necessary ; after some macro calls, which becomes invalid Rust syntax in the above input. The call x!(x); appears unaffected, regardless of the order of the two calls.

-     x!(-);
+     x!(-)

Self-contained repro as of current master (e0c7b7d):

$ echo 'extern { x!(-); x!(x); }' | cargo run --bin rustfmt
extern "C" {
    x!(-)
    x!(x);
}

$ !! | rustc -
error: macros that expand to items must be delimited with braces or followed by a semicolon
 --> <anon>:2:7
  |
2 |     x!(-)
  |       ^^^
  |
help: change the delimiters to curly braces
  |
2 |     x!{-}
  |       ~ ~
help: add a semicolon
  |
2 |     x!(-);
  |          +

[dtolnay]

This is important to https://github.com/dtolnay/cxx, which uses macro calls such as the following where the <>-based ones are affected by this bug:

#[cxx::bridge]
mod ffi {
    extern "C++" {
        include!("path/to/thing.h");    // turns into `#include "path/to/thing.h"` in C++
        include!(<some/systemheader>);   // turns into `#include <some/systemheader>`

        type Thing;
        fn whatever() -> UniquePtr<Thing>;
    }
}