Rollup of 22 pull requests

Successful merges:

 - #55391 (bootstrap: clean up a few clippy findings)
 - #56021 (avoid features_untracked)
 - #56023 (atomic::Ordering: Get rid of misleading parts of intro)
 - #56080 (Reduce the amount of bold text at doc.rlo)
 - #56114 (Enclose type in backticks for "non-exhaustive patterns" error)
 - #56124 (Fix small doc mistake on std::io::read::read_to_end)
 - #56127 (Update an outdated comment in mir building)
 - #56148 (Add rustc-guide as a submodule)
 - #56149 (Make std::os::unix/linux::fs::MetadataExt::a/m/ctime* documentation clearer)
 - #56220 (Suggest appropriate place for lifetime when declared after type arguments)
 - #56223 (Make JSON output from -Zprofile-json valid)
 - #56236 (Remove unsafe `unsafe` inner function.)
 - #56255 (Update outdated code comments in StringReader)
 - #56257 (rustc-guide has moved to rust-lang/)
 - #56273 (Add missing doc link)
 - #56289 (Fix small typo in comment of thread::stack_size)
 - #56294 (Fix a typo in the documentation of std::ffi)
 - #56312 (Deduplicate literal -> constant lowering)
 - #56319 (fix futures creating aliasing mutable and shared ref)
 - #56321 (rustdoc: add bottom margin spacing to nested lists)
 - #56322 (resolve: Fix false-positives from lint `absolute_paths_not_starting_with_crate`)
 - #56330 (Clean up span in non-trailing `..` suggestion)

Failed merges:

r? @ghost

Clean up span in non-trailing `..` suggestion

resolve: Fix false-positives from lint `absolute_paths_not_starting_with_crate`

Fixes https://github.com/rust-lang/rust/issues/56311 (stable-to-beta regression)

rustdoc: add bottom margin spacing to nested lists

the current CSS for nested lists sets margin-bottom to zero, which means that a nested list is squished up against subsequent list items/paragraphs

this changes the bottom margin to .6em, same as for paragraphs

before:
![before](https://user-images.githubusercontent.com/33935229/49176291-699ba800-f342-11e8-9d33-c4acd4df2beb.png)
after:
![after](https://user-images.githubusercontent.com/33935229/49176299-6f918900-f342-11e8-90c3-aa14db195b45.png)

fix futures creating aliasing mutable and shared ref

Fixes the problem described in https://github.com/solson/miri/issues/532#issuecomment-442552764: `set_task_waker` takes a shared reference and puts a copy into the TLS (in a `NonNull`), but `get_task_waker` gets it back out as a mutable reference. That violates "mutable references must not alias anything"!

Deduplicate literal -> constant lowering

Fix a typo in the documentation of std::ffi

Fix small typo in comment of thread::stack_size

Add missing doc link

r? @steveklabnik

rustc-guide has moved to rust-lang/

r? @nikomatsakis

Update outdated code comments in StringReader

For the detection of newlines in the lexer, this is now done in `analyze_source_file.rs`.

Remove unsafe `unsafe` inner function.

Within this `Iterator` implementation, a function `unsafe_get` is
defined which unsafely allows _unchecked_ indexing of any element in a
slice. This should be marked as _unsafe_, but it is not.

To address this issue, I removed that inner function.

Make JSON output from -Zprofile-json valid

r? @wesleywiser

cc https://github.com/rust-lang-nursery/rustc-perf/issues/299

Suggest appropriate place for lifetime when declared after type arguments

Make std::os::unix/linux::fs::MetadataExt::a/m/ctime* documentation clearer

I was confused by this API so I clarified what they are doing.

I was wondering if I should try to unify more documentation and examples between `unix` and `linux` (e.g. “of the file” is used in `unix` to refer to the file these metadata is for, “of this file” in `linux`, “of the underlying file” in `std::fs::File`).

Add rustc-guide as a submodule

Adding this as a submodule will allow two things:
- Linking to the guide from doc.rlo
- Doing a link check as part of the rust CI build

Key question: Do we want to wait for the book to be filled out more? e.g. do we ever want to move it out of the nursery?

r? @nikomatsakis

cc @steveklabnik

Update an outdated comment in mir building

r? @EddyB

Fix small doc mistake on std::io::read::read_to_end

The std::io::read main documentation can lead to error because the buffer is prefilled with 10 zeros that will pad the response.
Using an empty vector is better.

The `read_to_end` documentation is already correct though.

This is my first rust PR, don't hesitate to tell me if I did something wrong.

Enclose type in backticks for "non-exhaustive patterns" error

This makes the error style consistent with the convention in error messages.

Reduce the amount of bold text at doc.rlo

Currently, all of the text is either huge or small. IMHO this is hard to read, so I propose that we make the second-level headings smaller, without making them any less prominent.

# Before:

![image](https://user-images.githubusercontent.com/8827840/48737315-50447e80-ec13-11e8-8243-6211dbba1aa1.png)

# After:

![image](https://user-images.githubusercontent.com/8827840/48737258-32771980-ec13-11e8-814e-e7851954c05a.png)

atomic::Ordering: Get rid of misleading parts of intro

Remove the parts of atomic::Ordering's intro that wrongly claimed that
SeqCst prevents all reorderings around it.

Closes #55196

This is a (minimal) alternative to #55233.

I also wonder if it would be worth adding at least some warnings that atomics are often a footgun/hard to use correctly, similarly like `mem::transmute` or other functions have.

avoid features_untracked

The docs say to not use `features_untracked` when we have a tcx.

@oli-obk any particular reason why the untracked version is used all over const qualification?

bootstrap: clean up a few clippy findings

remove useless format!()s
remove redundant field names in a few struct initializations
pass slice instead of a vector to a function
use is_empty() instead of comparisons to .len()

No functional change intended.

Update LLVM

In particular this fixes #56265.

r? @alexcrichton

Stabilize feature `macro_at_most_once_rep`

a.k.a. `?` Kleene operator 🎉

cc #48075

r? @Centril

rustc_codegen_llvm: don't overalign loads of pair operands.

Counterpart to #56300, but for loads instead of stores.

Fix alignment of stores to scalar pair

The alignment for the second element of a scalar pair is not the same as for the first element, make sure it is calculated correctly. This fixes #56267.

r? @EddyB