rustfmt removes empty newlines between blocks of module-level attributes

In a situation like

#![feature(core_intrinsics)]
#![feature(lang_items)]
#![feature(libc)]
#![feature(nll)]
#![feature(panic_unwind)]
#![feature(staged_api)]
#![feature(std_internals)]
#![feature(unwind_attributes)]
#![feature(abi_thiscall)]
#![feature(rustc_attrs)]
#![feature(raw)]
#![panic_runtime]
#![feature(panic_runtime)]

// `real_imp` is unused with Miri, so silence warnings.
#![cfg_attr(miri, allow(dead_code))]

rustfmt removes that empty line. That's not great, as grouping these attributes by "paragraphs" conveys semantic meaning.

rustfmt preserves empty lines between statements, so it seems like the semantic value of that information is acknowledged, but it is not done consistently.

Start by reproducing the issue with the module-level attribute example in the report, then compare it with rustfmt's existing handling of empty lines between statements. Trace the formatting path for module-level attributes and locate the related formatter tests. Done means the blank line between attribute groups is preserved without changing the existing statement behavior.