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.
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.