Create symlinks to workaround file missing error in llvm-config

…ous trait predicate

…s, r=notriddle

`<details>`/`<summary>` UI fixes

With images it's easier to understand:

![Screenshot from 2022-05-21 14-10-42](https://user-images.githubusercontent.com/3050060/169653038-9c58de67-589a-4986-a8ff-dbdddaf136a4.png)
![Screenshot from 2022-05-21 14-08-49](https://user-images.githubusercontent.com/3050060/169653042-56e87258-13fe-4f80-9858-4e15c318c3fb.png)

The headings in `<summary>` should not have bottom border so I removed it as well alongside the other fixes.

r? `@jsha`

Allow macOS to build LLVM as shared library

Inspired by how [homebrew](https://github.com/Homebrew/homebrew-core/blob/HEAD/Formula/llvm.rb) builds and distributes llvm, here we manually create a symlink with a versioned dylib path to make `llvm-config` work properly. Note, the resulting `rustc` executable and `librustc_driver-<hash>.dylib` still links to the un-versioned `libLLVM.dylib` as expected when distributed in the final output. I have confirmed this by checking `otool -L` on both binaries.

After the change, enabling `llvm.link-shared` and `llvm.thin-lto` will be possible on macOS.

…r=jsha

Use CSS variables to handle theming

This is the start for our simplification of theming. Considering how big the diff quickly becomes, I think it's better to do it in multiple parts.

(The 3 first commits come from rust-lang#98297 so once it's merged, they'll disappear).

Normally they shouldn't be any UI changes. You can check it [here](https://rustdoc.crud.net/imperio/css-simplification/doc/foo/index.html).

cc `@notriddle`
r? `@jsha`

…, r=lcnr

Improve some inference diagnostics

- Properly point out point location where "type must be known at this point", or else omit the note if it's not associated with a useful span.
- Fix up some type ambiguity diagnostics, errors shouldn't say "cannot infer type for reference `&'a ()`" when the given type has no inference variables.

…llaumeGomez

rustdoc: fix 98690 Panic if invalid path for -Z persist-doctests

Closes rust-lang#98690 for rustdoc panic

I changed this to do eprintln and orderly panic instead of unwrap doing unhandled panic

~/gg/rust/build/x86_64-unknown-linux-gnu/stage2/bin/rustdoc --test -Z unstable-options --persist-doctests /tmp/foobar main.rs
Couldn't create directory for doctest executables: Permission denied (os error 13)