Several fixes for DragonFly (rebase)
load_self() needs to be public
debuginfo: Create debuginfo for for-loop variables again.
debuginfo: Clean the debuginfo module up a bit.
debuginfo: Add test case for destructured for-loop variable.
Support all variants of WherePredicate

Adds support for all variants of ast::WherePredicate in clean/mod.rs. Fixes #20048, but will need modification when EqualityPredicates are fully implemented in #20041.
rustdoc: unmarked code fences are doctests too
Address code review nits
rustdoc: render `for<>` on old closure lifetimes
rustdoc: render higher-rank trait bounds

Fix #19915
rustdoc: Use unboxed closure sugar

This unfortunately leaves sugaring Fn/FnMut/FnOnce on cross-crate
re-exports for future work.

cc #19909
rustdoc: Use Fn trait sugar for external paths

Fix #19909
auto merge of #19934 : tomjakubowski/rust/rustdoc-unboxed-closures-redux, r=alexcrichton

We render HRTB and the unboxed closure trait sugar (the so-called 
"parenthesized" notation) where appropriate. Also address the new
`for` syntax on the old closures.
auto merge of #20024 : mneumann/rust/dragonfly-fixes3, r=alexcrichton
auto merge of #20167 : michaelwoerister/rust/for-loop-var, r=alexcrichton

... really this time `:)`

I went for the simpler fix after all since it turned out to become a bit too complicated to extract the current iteration value from its containing `Option` with the different memory layouts it can have. It's also what we already do for match bindings.

I also extended the new test case to include the "simple identifier" case.

Fixes #20127, fixes #19732
auto merge of #20180 : jroesch/rust/clean-where-predicate, r=alexcrichton

Add support for all variants of ast::WherePredicate in clean/mod.rs. Fixes #20048, but will need modification when EqualityPredicates are fully implemented in #20041.
auto merge of #20183 : japaric/rust/doctests, r=alexcrichton

#20075 introduced a bug where unmarked code fences weren't considered as doctests. This PR fixes the logic.

---

This passed `check-stage1-rustdoc`, and I manually checked that:

``` rust
//! ```
//! println!("Hello")
//! ```
//!
//! ``` rust
//! println!("Hello")
//! ```
//!
//! ``` sh
//! println!("Hello")
//! ```
//!
//! ``` ignore
//! println!("Hello")
//! ```
```

Generated:

``` rust
running 3 tests
test _2 ... ignored
test _0 ... ok
test _1 ... ok
```

I'd love to add that as a test, but I have no idea how to do that with our testing infrastructure. If anyone knows how, do let me know!

r? @alexcrichton 
@seanmonstar feedback?