Drop support for LLVM < 10

[langston-barrett]

It's not clear that the FactGenerator would compile with LLVM less than 10, and testing this (e.g., in CI) would add a maintenance burden for no clear benefit. LLVM 10 is quite old, there are unlikely to be clients interested in it.

[langston-barrett]

The docs only claim compatibility with LLVM 10-16, so fixing this issue just requires removing old #if LLVM_VERSION_MAJORs from the FactGenerator.