Update for LLVM 23

vadrequested to merge
vad.sol/llvm-sys.rs:vad/llvm-23 into main

Since the API changes are based on LLVM 23.1.0-rc2, this is currently tagged as a prerelase (-rc2) crate.

The https://apt.llvm.org/llvm.sh script does not work properly for LLVM 23, even though the packages are available, so configure the APT repository and install necessary packages manually.

Edited by vad

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