Tracking issue for rustfmt bugs affecting CXX

[dtolnay]

1. ~Fails with "invalid ABI" on extern "C++" block — https://github.com/rust-lang/rustfmt/issues/4086~
2. ~Replaces any extern "C++" block with extern "Rust" — https://github.com/rust-lang/rustfmt/issues/4086#issuecomment-601337802~
   • Fixed https://github.com/rust-lang/rustfmt/pull/4088
   • Backport to rustfmt 1.x https://github.com/rust-lang/rustfmt/pull/4089
3. ~Deletes rhs from extern type declaration — https://github.com/rust-lang/rustfmt/issues/4159~
   • Fixed https://github.com/rust-lang/rustfmt/pull/4164
   • Backport to rustfmt 1.x https://github.com/rust-lang/rustfmt/pull/4447
4. ~Fails on input containing unsafe extern or unsafe mod~
   • Fixed https://github.com/rust-lang/rustfmt/pull/4433
   • Backport to rustfmt 1.x https://github.com/rust-lang/rustfmt/pull/4434
   • Integration into Rust 1.49 https://github.com/racer-rust/racer/pull/1132, https://github.com/rust-lang/rls/pull/1701, https://github.com/rust-lang/rust/pull/77590
   • Backport to Rust 1.48 https://github.com/rust-lang/rust/pull/77705
5. ~Deletes unsafe keyword on extern fn signatures (among other things) — https://github.com/rust-lang/rustfmt/issues/4288~
   • Fixed https://github.com/rust-lang/rustfmt/pull/4291
   • Backport to rustfmt 1.x https://github.com/rust-lang/rustfmt/pull/4562
   • Landed in Rust 1.50