rustfmt removes attributes on parameters of `fn` types

[ChayimFriedman2]

I formatted this code:

struct Foo {
    v: fn(#[cfg(false)] i32),
}

Playground.

I expected to see this happen: rustfmt keeps the attribute, it shouldn't change the meaning of the program.

Instead, this happened: rustfmt turns this into:

struct Foo {
    v: fn(i32),
}

@rustbot label +A-rustfmt

[fmease]

@rustbot transfer rustfmt

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