Make meta-item API compatible with `LocalInternedString::get` soundness fix

r? @Zoxc

make asm diagnostic instruction optional

`DiagnosticInfoInlineAsm::getInstruction` may return a null pointer, so
the instruction shouldn't be blindly unwrapped.

Reopening from #55193. I was unable to trigger the assertion on Windows after rebasing.

Fixes #23458.
Fixes #55216.

use the identifier span for missing struct field

When moving out of a for loop head, suggest borrowing it

When encountering code like the following, suggest borrowing the for loop
head to avoid moving it into the for loop pattern:

```
fn main() {
    let a = vec![1, 2, 3];
    for i in &a {
        for j in a {
            println!("{} * {} = {}", i, j, i * j);
        }
    }
}
```

Fix #25534.

Separate `DefId`s for variants and their constructors

Part of #44109. Split off from #59376. See [Zulip topic](https://rust-lang.zulipchat.com/#narrow/stream/131828-t-compiler/topic/rfc-2008/near/132663140) for previous discussion.

r? @petrochenkov

When encountering code like the following, suggest borrowing the for loop
head to avoid moving it into the for loop pattern:

```
fn main() {
    let a = vec![1, 2, 3];
    for i in &a {
        for j in a {
            println!("{} * {} = {}", i, j, i * j);
        }
    }
}
```

This commit moves the `DefId` field of `Def::Ctor` to be the first
field.

This commit moves the definition of `CtorOf` from `rustc::hir` to
`rustc::hir::def` and adds imports wherever it is used.

Rollup of 7 pull requests

Successful merges:

 - #59213 (Track changes to robots.txt)
 - #59239 (Remove inline assembly from hint::spin_loop)
 - #59251 (Use a valid name for graphviz graphs)
 - #59296 (Do not encode gensymed imports in metadata)
 - #59328 (Implement specialized nth_back() for Box and Windows.)
 - #59355 (Fix ICE with const generic param in struct)
 - #59377 (Correct minimum system LLVM version in tests)

This commit removes `CtorOf` from `Node::Ctor` as the parent of the
constructor can be determined by looking at the node's parent in the few
places where knowing this is necessary.

`DiagnosticInfoInlineAsm::getInstruction` may return a null pointer, so
the instruction shouldn't be blindly unwrapped.

Correct minimum system LLVM version in tests

Since commit 9452a8df, the new debug info format is only generated
for LLVM 8 and newer versions. However, the tests still assume that LLVM
7 will use the new debug info format. Fix the tests (and a comment in
the code) to match the actual version check.

(WIP) Small fixes in chalkification

Small fixes around region constraints and builtin impls. There are still some type inference errors, for example the following code errors out:
```rust
fn main() {
    let mut x: Vec<i32> = Vec::new();
    //                    ^^^^^^^^ cannot infer type for `std::vec::Vec<_>`
}
```
but explicitly specifying `Vec::<i32>::new` works.

With these few fixes, the following code now passes type-checking:
```rust
fn main() {
    let mut x: Vec<i32> = Vec::<i32>::new();
    x.push(5);
    println!("{:?}", x);
}
```

I also fixed the implied bounds bug as discussed on Zulip and in https://github.com/rust-lang-nursery/chalk/pull/206

cc @tmandry
r? @nikomatsakis

Fix ICE with const generic param in struct

Fixes https://github.com/rust-lang/rust/issues/59340.

r? @petrochenkov

Implement specialized nth_back() for Box and Windows.

Hi there, this is my first pull request to rust :-)

I started implementing some specializations for DoubleEndedIterator::nth_back() and these are the first two. The problem has been discussed in #54054 and nth_back() is tracked in #56995.

I'm stuck with the next implementation so I though I do a PR for the ones I'm confident with to get some feedback.

Do not encode gensymed imports in metadata

(Unless they are underscore `_` imports which are re-gensymed on crate loading, see https://github.com/rust-lang/rust/pull/56392.)

We cannot encode gensymed imports properly in metadata and if we encode them improperly, we can get erroneous name conflicts downstream.
Gensymed imports are produced by the compiler, so we control their set, and can be sure that none of them needs being encoded for use from other crates.

A workaround that fixes https://github.com/rust-lang/rust/issues/59243.

Use a valid name for graphviz graphs

Hiridification has broken graphviz output because `HirId` has a more complex display implemetation than `NodeId`. Since the id was just used to generate a distinct identifier, we just pull out the various constituent indexed.

This commit makes two changes - separating the `NodeId` that identifies
an enum variant from the `NodeId` that identifies the variant's
constructor; and no longer creating a `NodeId` for `Struct`-style enum
variants and structs.

Separation of the variant id and variant constructor id will allow the
rest of RFC 2008 to be implemented by lowering the visibility of the
variant's constructor without lowering the visbility of the variant
itself.

No longer creating a `NodeId` for `Struct`-style enum variants and
structs mostly simplifies logic as previously this `NodeId` wasn't used.
There were various cases where the `NodeId` wouldn't be used unless
there was an unit or tuple struct or enum variant but not all uses of
this `NodeId` had that condition, by removing this `NodeId`, this must
be explicitly dealt with. This change mostly applied cleanly, but there
were one or two cases in name resolution and one case in type check
where the existing logic required a id for `Struct`-style enum variants
and structs.

Remove inline assembly from hint::spin_loop

This PR removes the inline assembly which was not required since these
instructions are available in core::arch, and extends support of
the spin_loop hint to arm targets with the v6 feature which also
support the yield instruction.

Track changes to robots.txt

Currently `robots.txt` of doc.rust-lang.org is not part of any repo, so there's [no way to contribute any changes to it](https://internals.rust-lang.org/t/deadlock-about-fixing-outdated-documentation-links-in-search-engines/9374), such as needed for #44894 and countless dupes of this issue.

I propose adding it to this repo. I'm not in control of the infrastructure, so I can't help to automate deployment of it, but even just having the file under source control is IMHO a step forward.

Remove `track_errors` from `check_match`, `typeck_item_bodies` and `register_plugins`

In the spirit of continuing through errors in type checking (#39275), remove `track_errors` from a couple of locations in the codebase.

Tweak some diagnostic spans

HirIdification: kill off NodeId stragglers

The final stages of HirIdification (#57578).

This PR, along with https://github.com/rust-lang/rust/pull/59042, should finalize the HirIdification process (at least the more straightforward bits).

- replace `NodeId` with `HirId` in `trait_impls`
- remove all `NodeId`s from `borrowck`
- remove all `NodeId`s from `typeck`
- remove all `NodeId`s from `mir`
- remove `trait_auto_impl` (unused)

I would be cool to also remove `NodeId` from `hir::def::Def`, `middle::privacy::AccessLevel`  and `hir::ItemId`, but I don't know if this is feasible.

I'll be happy to do more if I've missed anything.