Support for LLVM 16

[athas]

Oclgrind does not seem to support LLVM 16 yet. The first showstopper is that clang has changed the name of the include directory that contains opencl-c.h. From the release notes:

Clang’s resource directory path previously included the full clang version. It now includes only the major version. The new resource directory is $prefix/lib/clang/$CLANG_MAJOR_VERSION and can be queried using clang -print-resource-dir, just like before.

[jrprice]

LLVM 15 isn't yet supported either.

I have WIP changes to migrate to opaque pointers which is the main blocker on upgrading. They're almost done, so I will do my best to find some time in the near future to get that finished off so that we can support LLVM 15 and 16. Sorry for the delays!