LLVM Versions

[peterhenryd]

It's likely that support for a variety of LLVM versions will be wanted in the future; however, I'm unsure whether it's necessary since Dorian is a very high-level library and being able to switch LLVM versions with ease might be more work than it's worth; however, for the purpose of easily upgrading the library in the future as new versions of LLVM are released, it is likely necessary.

Since Dorian already compartmentalizes and directly abstracts LLVM, the main thing would be configuring cargo and features to work as desired, as well as adding macros to generate versioned LLVM functions.