Comments on items in `extern` block are deleted

[Aaron1011]

Describe the bug

When a normal comment (non-doc-comment) is applied to an item in an extern block, rustfmt will delete it.

To Reproduce

The following code:

extern "C" {
    // A comment
    fn bar();
}

is formatted as:

extern "C" {
    fn bar();
}

Expected behavior

rustfmt should leave the comment intact

Meta

• rustfmt version: rustfmt 1.4.26-nightly (293d7d0 2020-11-14)
• From where did you install rustfmt?: rustup
• How do you run rustfmt: rustfmt

[calebcartwright]

Duplicate of #4528, fixed in the next nightly (rust-lang/rust#79120)