Tracking issue for rustfmt bugs affecting CXX

[dtolnay]

1. Fails with "invalid ABI" on extern "C++" block — Allow arbitrary extern ABI rust-lang/rustfmt#4086
2. Replaces any extern "C++" block with extern "Rust" — Allow arbitrary extern ABI rust-lang/rustfmt#4086 (comment)
   • Fixed fix formatting arbitrary extern abi rust-lang/rustfmt#4088
   • Backport to rustfmt 1.x [rustfmt 1.x] fix formatting arbitrary extern abi rust-lang/rustfmt#4089
3. Deletes rhs from extern type declaration — Rustfmt destroys extern types that have generics or rhs rust-lang/rustfmt#4159
   • Fixed Preserve and format type aliases in extern blocks rust-lang/rustfmt#4164
   • Backport to rustfmt 1.x 1.x: Preserve and format type aliases in extern blocks rust-lang/rustfmt#4447
4. Fails on input containing unsafe extern or unsafe mod
   • Fixed Bump rustc-ap to v679 rust-lang/rustfmt#4433
   • Backport to rustfmt 1.x 1.x: Bump rustc-ap to v679 rust-lang/rustfmt#4434
   • Integration into Rust 1.49 bump rustc-ap* to v679 racer-rust/racer#1132, bump racer and rustfmt versions rust-lang/rls#1701, Update RLS and Rustfmt rust-lang/rust#77590
   • Backport to Rust 1.48 [beta] backports rust-lang/rust#77705
5. Deletes unsafe keyword on extern fn signatures (among other things) — Async gets removed from extern blocks. rust-lang/rustfmt#4288
   • Fixed Do not remove fn headers (e.g., async) on extern fn items rust-lang/rustfmt#4291
   • Backport to rustfmt 1.x 1.x: Backport fixes around deleting pieces of extern functions rust-lang/rustfmt#4562
   • Landed in Rust 1.50