Backports (Upstream Not Supported) and New LLVM Release Updates

cherry-picked directly to our fork. 1. Make sure the bugfix is in upstream LLVM. 2. Identify the branch that rustc is currently using. The `src/llvm-project` submodule is always pinned to a branch of the [rust-lang/llvm-project repository]. 3. Fork the rust-lang/llvm-project repository. 4. Check out the appropriate branch (typically named `rustc/a.b-yyyy-mm-dd`). 5. Add a remote for the upstream repository using `git remote add upstream https://github.com/llvm/llvm-project.git` and fetch it using `git fetch upstream`. 6. Cherry-pick the relevant commit(s) using `git cherry-pick -x`. 7. Push this branch to your fork. 8. Send a Pull Request to rust-lang/llvm-project to the same branch as before. Be sure to reference the Rust and/or LLVM issue that you're fixing in the PR description. 9. Wait for the PR to be merged. 10. Send a PR to rust-lang/rust updating the `src/llvm-project` submodule with your bugfix. This can be done locally with `git submodule update --remote src/llvm-project` typically. 11. Wait for PR to be merged. An example PR: [#59089](https://github.com/rust-lang/rust/pull/59089) ## New LLVM Release Updates  Unlike bugfixes, updating to a new release of LLVM typically requires a lot more work. This is where we can't reasonably cherry-pick commits backwards, so we need to do a full update. There's a lot of stuff to do here, so let's go through each in detail. 1. LLVM announces that its latest release version has branched. This will show up as a branch in the [llvm/llvm-project repository], typically named `release/$N.x`, where `$N` is the version of LLVM that's being released. 1. Create a new branch in the [rust-lang/llvm-project repository] from this `release/$N.x` branch, and name it `rustc/a.b-yyyy-mm-dd`, where `a.b` is the current version number of LLVM in-tree at the time of the branch, and the remaining part is the current date. 1. Apply Rust-specific patches to the llvm-project repository. All features and bugfixes are upstream, but there's often some weird build-related patches that don't make sense to upstream. These patches are typically the latest patches in the rust-lang/llvm-project branch that rustc is currently using. 1. Build the new LLVM in the `rust` repository. To do this, you'll want to update the `src/llvm-project` repository to your branch, and the revision you've created. It's also typically a good idea to update `.gitmodules` with the new branch name of the LLVM submodule. Make sure you've committed changes to `src/llvm-project` to ensure submodule updates aren't reverted. Some commands you should execute are: * `./x build src/llvm-project` - test that LLVM still builds * `./x build` - build the rest of rustc You'll likely need to update [`llvm-wrapper/*.cpp`][`llvm-wrapper`] to compile with updated LLVM bindings. Note that you should use `#ifdef` and such to ensure that the bindings still compile on older LLVM versions. Note that `profile = "compiler"` and other defaults set by `./x setup` download LLVM from CI instead of building it from source. You should disable this temporarily to make sure your changes are being used.

This section describes how to update the LLVM version used by Rust when upstream support for backports is no longer available, involving cherry-picking commits directly to the Rust fork. Additionally, it details the process for updating to a new major LLVM release, including creating a new branch in the rust-lang/llvm-project repository, applying Rust-specific patches, building the new LLVM within the Rust repository, and updating the llvm-wrapper files. It emphasizes the need to test builds and ensure compatibility with older LLVM versions.