Remove the default type of `Rem::Output`

Associated type defaults are not yet stable, and `Rem` is the only trait that specifies a default. Let's see what breaks when it's removed.

cc https://github.com/rust-lang/rust/pull/61812#issuecomment-502394566
cc @Centril

@bors try

librustc_data_structures: Speedup union of sparse and dense hybrid set

This optimization speeds up the union of a hybrid bitset when that
switches it from a sparse representation to a dense bitset. It now
clones the dense bitset and integrate only the spare elements instead of
densifying the sparse bitset, initializing all elements, and then a
union on two dense bitset, touching all words a second time.

It's not completely certain if the added complexity is worth it but I would
like to hear some feedback in any case. Benchmark results from my machine:

```
Now:  bit_set::union_hybrid_sparse_to_dense ... bench:          72 ns/iter (+/- 5)
Previous: bit_set::union_hybrid_sparse_to_dense ... bench:          90 ns/iter (+/- 6)
```

This being the second iteration of trying to improve the speed, since I missed the return value in the first, and forgot to run the relevant tests. Oops.

update miri

r? @oli-obk

Kill conflicting borrows of places with projections.

Resolves #62007.

Due to a bug, the previous version of this check did not actually kill all conflicting borrows unless the borrowed place had no projections. Specifically, `sets.on_entry` will always be empty when `statement_effect` is called. It does not contain the set of borrows which are live at this point in the program.

@pnkfelix describes why this was not caught before in #62007, and created an example where the current borrow checker failed unnecessarily. This PR adds their example as a test, but they will likely want to add some additional ones.

r? @pnkfelix

Rollup of 9 pull requests

Successful merges:

 - #60971 (Add DocFS layer to rustdoc)
 - #61146 (SliceConcatExt::connect defaults to calling join)
 - #61181 (Fix theme-checker failure)
 - #61267 (rustc-book: Update the rustc/clang compatibility table for xLTO.)
 - #61270 (Remove warnings about incr. comp. generating less debugging output.)
 - #61681 (Changed the error message to more clearly explain what is allowed)
 - #61984 (More NodeId pruning)
 - #62016 (Add test for issue-27697)
 - #62019 (Remove needless lifetimes)

Failed merges:

r? @ghost

Remove needless lifetimes

Add test for issue-27697

Closes #27697

More NodeId pruning

Just another round of the `HirId`ification initiative.

r? @Zoxc

Changed the error message to more clearly explain what is allowed

This is in regard to #61634. I changed the language to make it more clear what is allowed.