…e they are no longer supported by llvm

CMake Warning:
  Manually-specified variables were not used by the project:

    PYTHON_EXECUTABLE
    WITH_POLLY

The code in `ucred.rs` is based on the work done in PR 13 in the
tokio-uds repository on GitHub. Link below for reference:

    tokio-rs/tokio-uds#13

Credit to Martin Habovštiak (GitHub username Kixunil) and contributors
for this work!

We can simply init the struct directly. There is no real need to use
uninit memory here.

Co-authored-by: lzutao <taolzu@gmail.com>

Currently, PID will be populated for Linux, and set to None for BSDs.

The only link that I did not change is a link to a function on the
`pointer` primitive because intra-doc links for the `pointer` primitive
don't work yet (see rust-lang#63351).

Many developers use a dark theme with editors and IDEs, but this
typically doesn't extend to graphviz output.

When I bring up a MIR graphviz document, the white background is
strikingly bright. This new option changes the colors used for graphviz
output to work better in dark-themed UIs.

`write` is ambiguous because there's also a macro called `write`.

Also removed unnecessary and potentially confusing link to a function in
its own docs.

#safety -> self#safety

VS code graphviz extensions use d3-graphviz, which supports `Courier`
fontname but does not support `monospace`. This caused graphs to render
poorly because the text sizes were wrong.

…etrochenkov"

This reverts commit 85cee57, reversing
changes made to b4d3873.

Move `rustllvm` into `compiler/rustc_llvm`

The `rustllvm` directory is not self-contained, it contains C++ code built by a build script of the `rustc_llvm` crate which is then linked into that crate.
So it makes sense to make `rustllvm` a part of `rustc_llvm` and move it into its directory.
I replaced `rustllvm` with more obvious `llvm-wrapper` as the subdirectory name, but something like `llvm-adapter` would work as well, other suggestions are welcome.

To make things more confusing, the Rust side of FFI functions defined in `rustllvm` can be found in `rustc_codegen_llvm` rather than in `rustc_llvm`. Perhaps they need to be moved as well, but this PR doesn't do that.

The presence of multiple LLVM-related directories in `src` (`llvm-project`, `rustllvm`, `librustc_llvm`, `librustc_codegen_llvm` and their predecessors) historically confused me and made me wonder about their purpose.
With this PR we will have LLVM itself (`llvm-project`), a FFI crate (`rustc_llvm`, kind of `llvm-sys`) and a codegen backend crate using LLVM through the FFI crate (`rustc_codegen_llvm`).

Implementation of peer credentials for Unix sockets

The code in `ucred.rs` is based on the work done in [PR 13](tokio-rs/tokio-uds#13) in the tokio-uds repository on GitHub.

This commit is effectively a port to the stdlib, so credit to Martin Habovštiak (@Kixunil) and contributors for the meat of this work. 🥇

Happy to make changes as needed. 🙂

…k-Simulacrum

rustbuild: don't set PYTHON_EXECUTABLE and WITH_POLLY cmake vars since they are no longer supported by llvm

This resolves

CMake Warning:
  Manually-specified variables were not used by the project:

    PYTHON_EXECUTABLE
    WITH_POLLY

…r, r=jyn514

Use intra-doc links in `core::ptr`

Part of rust-lang#75080.

The only link that I did not change is a link to a function on the
`pointer` primitive because intra-doc links for the `pointer` primitive
don't work yet (see rust-lang#63351).

---

@rustbot modify labels: A-intra-doc-links T-doc

Add -Zgraphviz_dark_mode and monospace font fix

Many developers use a dark theme with editors and IDEs, but this
typically doesn't extend to graphviz output.

When I bring up a MIR graphviz document, the white background is
strikingly bright. This new option changes the colors used for graphviz
output to work better in dark-themed UIs.

<img width="1305" alt="Screen Shot 2020-09-09 at 3 00 31 PM" src="https://user-images.githubusercontent.com/3827298/92659478-4b9bff00-f2ad-11ea-8894-b40d3a873cb9.png">

Also fixed the monospace font for common graphviz renders (e.g., VS Code extensions), as described in rust-lang#76500 (comment)

**Before:**
<img width="943" alt="Screen Shot 2020-09-09 at 2 48 44 PM" src="https://user-images.githubusercontent.com/3827298/92658939-47231680-f2ac-11ea-97ac-96727e4dd622.png">

**Now with fix:**
<img width="943" alt="Screen Shot 2020-09-09 at 2 49 02 PM" src="https://user-images.githubusercontent.com/3827298/92658959-51451500-f2ac-11ea-9aae-de982d466d6a.png">

…ulacrum

Document btree's unwrap_unchecked

rust-lang#74693's second wind

Revert rust-lang#76285

Fixes rust-lang#76399. Reverting because the issue is P-critical and there are no PRs up to fix it.

r? @Mark-Simulacrum
cc @matklad @dtolnay