Revert "debuginfo: Add test case for destructured for-loop variable."

This reverts commit 87c5927b79b8eec8763659da8a3cf4561a4ceabd.
Revert "debuginfo: Clean the debuginfo module up a bit."

This reverts commit 34a6fcf19566e5015c1ef4c144a408a2f182cf4d.
Revert "debuginfo: Create debuginfo for for-loop variables again."

This reverts commit b0481147184a8ee70f066423dc077ffa0bd821b5.
auto merge of #20132 : alexcrichton/rust/revert-20027, r=sfackler

The introduced issue https://github.com/rust-lang/rust/issues/20127 seems serious enough that we may want to enqueue a reversion for now. Feel free to beat me with a fix though @michaelwoerister!
Require types to opt-in Sync
Rename `OwnedPtr` to `UniquePtr`
Make Send and Sync traits unsafe
Move RacyCell to `std::comm`

RacyCell is not exactly what we'd like as a final implementation for
this. Therefore, we're moving it under `std::comm` and also making it
private.
Implement `Sync` for some windows sys types
Rename `UniquePtr` to `Unique`

Mostly following the convention in RFC 356
Relax `Arc` bounds don't require Sync+Send

Besides the above making sense, it'll also allow us to make `RacyCell`
private and use UnsafeCell instead.