LLVM 15

[langston-barrett]

Fixes #12. This may require changes to the Datalog code due to opaque pointers (#113).

[langston-barrett]

Yep, some Datalog assertions are failing. I think there are a few options:

• Try to fix the Datalog in this MR. This would be great, but could be a ton of effort.
• Ask users to compile their code with -Xclang -no-opaque-pointers. I tried this and it didn't seem to fix the tests, so might be a no-go.
• Wait until fixing dl: Adapt to opaque pointers #113 before supporting LLVM 15.
• Keep LLVM 14 as the default, but support compiling against LLVM 15. I like this because the FactGenerator is independently useful to have building against more recent LLVM.