Add tests that integrate with `clang -emit-llvm` output

[hryx]

Add tests that call clang -emit-llvm -c somefile.c to produce a .bc bitcode file, then parse that file with this library. This test can be orchestrated by build.zig.

It would be cool to do some fuzz testing by generating several C files of various sizes using https://github.com/travisstaloch/markov-chains-zig.