feat(llvm): migrate from LLVM 14 to LLVM 20 with updated API and impo…

…rt fixes

- Updated `LLVMBuildGEP2` and `LLVMBuildLoad2` to include required `LLVMTypeRef` arguments
- Replaced `LLVMBuildTrunc2` with `LLVMBuildTrunc`
- Removed obsolete `LLVMPassBuilderOptionsSetOptLevel` call
- Fixed `LLVMRunPasses` error handling with `LLVMGetErrorMessage`
- Removed unnecessary `unsafe` blocks in `optimise_ir`
- Clarified lifetime syntax in `execute` function
- Corrected imports for `llvm_sys` modules to resolve unresolved symbols
- Fixed type mismatches for `LLVMTargetRef` and other types