Add support for LLVM 18.

Add support for LLVM 17.

Fix FFI-related safety issue.

Add support for LLVM 16.

Add support for LLVM 15 and the following callbacks in PassBuilder:

• Optimizer early extension point callback
• Full LTO last extension point callback
• Full LTO early extension point callback

Add support for the following callbacks in PassBuilder:

• Peephole extension point callback
• Scalar optimizer late extension point callback
• Vectorizer start extension point callback
• Pipeline start extension point callback
• Pipeline early simplification extension point callback
• Optimizer late extension point callback

Breaking change: Initialize LLVM plugins with PassBuilder API (similar to the C++ LLVM API).

• Fix docs.rs build with LLVM version checking in build script

• Add utils iterators
• Fallback to using LLVM_SYS_*_PREFIX env var

Support - in pass names