refactor: add rustc-perf submodule to src/tools

Currently, it's very challenging to perform a sandboxed `opt-dist`
bootstrap because the tool requires `rustc-perf` to be present, but
there is no proper management/tracking of it. Instead, a specific commit
is hardcoded where it is needed, and a non-checksummed zip is fetched
ad-hoc. This happens in two places:

`src/ci/docker/host-x86_64/dist-x86_64-linux/Dockerfile`:

```dockerfile
ENV PERF_COMMIT 4f313ad
RUN curl -LS -o perf.zip https://ci-mirrors.rust-lang.org/rustc/rustc-perf-$PERF_COMMIT.zip && \
    unzip perf.zip && \
    mv rustc-perf-$PERF_COMMIT rustc-perf && \
    rm perf.zip
```

`src/tools/opt-dist/src/main.rs`

```rust
// FIXME: add some mechanism for synchronization of this commit SHA with
// Linux (which builds rustc-perf in a Dockerfile)
// rustc-perf version from 2023-10-22
const PERF_COMMIT: &str = "4f313add609f43e928e98132358e8426ed3969ae";

let url = format!("https://ci-mirrors.rust-lang.org/rustc/rustc-perf-{PERF_COMMIT}.zip");
let client = reqwest::blocking::Client::builder()
    .timeout(Duration::from_secs(60 * 2))
    .connect_timeout(Duration::from_secs(60 * 2))
    .build()?;
let response = retry_action(
    || Ok(client.get(&url).send()?.error_for_status()?.bytes()?.to_vec()),
    "Download rustc-perf archive",
    5,
)?;
```

This causes a few issues:

1. Maintainers need to be careful to bump PERF_COMMIT in both places
   every time
2. In order to run `opt-dist` in a sandbox, you need to provide your own
   `rustc-perf` (rust-lang#125125), but to
   figure out which commit to provide you need to grep the Dockerfile
3. Even if you manage to provide the correct `rustc-perf`, its
   dependencies are not included in the `vendor/` dir created during
   `dist`, so it will fail to build from the published source tarballs
4. It is hard to provide any level of automation around updating the
   `rustc-perf` in use, leading to staleness

Fundamentally, this means `rustc-src` tarballs no longer contain
everything you need to bootstrap Rust, and packagers hoping to leverage
`opt-dist` need to go out of their way to keep track of this "hidden"
dependency on `rustc-perf`.

This change adds rustc-perf as a git submodule, pinned to the current
`PERF_COMMIT` 4f313ad. Subsequent
commits ensure the submodule is initialized when necessary, and make use
of it in `opt-dist`.

.gitmodules

@@ -43,3 +43,7 @@
 	path = library/backtrace
 	url = https://github.com/rust-lang/backtrace-rs.git
 	shallow = true
+[submodule "src/tools/rustc-perf"]
+	path = src/tools/rustc-perf
+	url = https://github.com/rust-lang/rustc-perf.git
+	shallow = true

src/tools/tidy/config/black.toml

@@ -11,5 +11,6 @@ extend-exclude = """(\
     src/doc/edition-guide/|\
     src/llvm-project/|\
     src/doc/embedded-book/|\
+    src/tools/rustc-perf/|\
     library/backtrace/
     )"""

src/tools/tidy/config/ruff.toml

@@ -26,6 +26,7 @@ extend-exclude = [
     "src/llvm-project/",
     "src/doc/embedded-book/",
     "library/backtrace/",
+    "src/tools/rustc-perf/",
     # Hack: CI runs from a subdirectory under the main checkout
     "../src/doc/nomicon/",
     "../src/tools/cargo/",
@@ -38,4 +39,5 @@ extend-exclude = [
     "../src/llvm-project/",
     "../src/doc/embedded-book/",
     "../library/backtrace/",
+    "../src/tools/rustc-perf/",
 ]

src/tools/tidy/src/walk.rs

@@ -18,6 +18,7 @@ pub fn filter_dirs(path: &Path) -> bool {
         "src/tools/clippy",
         "src/tools/miri",
         "src/tools/rust-analyzer",
+        "src/tools/rustc-perf",
         "src/tools/rustfmt",
         "src/doc/book",
         "src/doc/edition-guide",