LLVM: Bump to LLVM 18

[XChy]

Fixes #453
If possible, I prefer to creating a new branch llvm-latest to cover it. But I have not writing access.
LLVM's IR and API have changed a lot so far, like opaque-ptr, and the master branch of dg is not compatible with latest LLVM version. So I post this PR to fix it.

[mchalupa]

Thanks a lot, such patches are always very welcome! I created the branch llvm-latest as you wanted. If you're OK with that, I would later apply this PR with some modifications to devel and eventually to master branches. (the modification would be just to drop reformatting of CMakeLists.txt and fix the changes so that they work with older LLVMs too).

[XChy]

Thanks. That's OK. But to note, this patch just makes it compilable, not free of bugs. Such integration with latest LLVM is untested, so we may need to fix it in the future.