Add support for LLVM 21.1

[b1scoito]

https://github.com/llvm/llvm-project/releases/tag/llvmorg-21.1.0

Newest Nightly toolchain uses it:

❯ rustc -Vv
rustc 1.91.0-nightly (07d246fc6 2025-08-31)
binary: rustc
commit-hash: 07d246fc6dc227903da2955b38a59e060539a485
commit-date: 2025-08-31
host: aarch64-apple-darwin
release: 1.91.0-nightly
LLVM version: 21.1.0

Contributing would be awesome, would you think that to update it to LLVM 21 is just a matter of replicating your LLVM 20 update or are there any other caveats?

[jamesmth]

It's a bit more complex. First, inkwell must support LLVM 21 and expose a cargo feature for it (something like llvm21-0).

Then, the CI workflow must be updated to test the new LLVM version. For that, a customized version of the LLVM toolchain (for Windows support) must be released here.

If you are motivated (provided inkwell supports LLVM 21), you could replicate the LLVM 20 update without much trouble. In the meantime, I will take care of releasing the new toolchain for the CI.

[stevefan1999-personal]

@jamesmth Indeed inkwell has supported LLVM 21 right now. As I cannot find any github actions workflow files, can you do the CI update?