Update for LLVM 21.1.0

LLVM 21.1.0-rc1 was published a few days ago and the final 21.1.0 release will follow in around 6 weeks.

---

For new LLVM major versions:

• Diff llvm/include/llvm-c/ between previous and current release. This requires a local repo that's in sync with the remote, and tags will be needed. A bare repository is fine, however.

  ```
  git clone --mirror https://github.com/llvm/llvm-project.git
  git diff llvmorg-9.0.0 llvmorg-10.0.0 -- llvm/include/llvm-c/
  ```
  
  Apply matching changes to Rust bindings.

• Update links key in Cargo.toml for new LLVM version

• Update usage example in README.md for new crate version

• Update CI to refer to new version

  • That includes an update of RUSTC_COMMIT variable to reference a commit in github.com/rust-lang/rust providing the new LLVM version. It can be determined using ./scripts/rustc-commit.sh. If preferred, it can be also checked in a local Rust repository clone:

    ```
    git clone --mirror https://github.com/rust-lang/rust.git
    git log --oneline --grep=LLVM --grep=$LLVM_VERSION main -- .gitmodules
    ```

For all versions:

• Update version key in Cargo.toml for new crate version
• Commit changes
• Tag new version; git tag v100.1.0
• Update latest branch to follow master: git branch -f llvm-10.0 master
• Test and publish
• Push changes and tags; git push --all && git push --tags