There are two things that make running MLIR tests with ASan on a mac
tedious:
1. The `DYLD_INSERT_LIBRARIES` environment variable needs to be set to
point to `libclang_rt.asan_osx_dynamic.dylib`
2. Mac is wrapping Python, which means that the `DYLD_INSERT_LIBRARIES`
environment variable is not being respected in the Python tests. The
solution is to find and use a non-wrapped Python binary.
With the above two changes, ASan works out of the box on mac's by
setting the `-DLLVM_USE_SANITIZER=Address` cmake flag.
I have stolen most of the code in this PR from other LLVM projects. It
may be a good idea to reconcile it somewhere.
