Remove TritonToLLVM. (#791)

Initial import of Jasc. (#790)

This PR imports Jasc, an MLIR-based compiler for JAX kernels, to the
repo. Most functionality works, but some doesn't, and some shortcuts
have been taken. In particular:

* The GPU code are not part of the import and the corresponding
  tests are deactivate. Maybe we need a better mechanism to choose
  the backend before we can make GPUs work in OSS.
* I have completely removed support for sparse and the related tests.
* I have removed several tools that aren't supported in OSS.
* I have also not spend a lot of effort to reduce the diff between this
  repository and our internal version but there are a few easy things we
  could do to make potential future syncs easier.

What does work, though, is to compile everything and run the tests
(some of which are deactivated):

    bazel build //...
    bazel test test:*